Nomenclature of Organic Chemistry. IUPAC Recommendations and Preferred Names 2013.

P-20 Introduction
P-21 Mononuclear and acyclic polynuclear parent hydrides
P-22 Monocyclic parent hydrides
P-23 Polyalicyclic parent hydrides (extended von Baeyer system)
P-24 Spiro ring systems
P-25 Fused and bridged fused ring systems
P-26 Phane nomenclature
P-27 Fullerenes
P-28 Ring assemblies
P-29 Prefixes denoting substituent groups derived from parent hydrides

A parent hydride is the structure that is named before the addition of affixes denoting substituents to yield the name of a specific compound. Its name is understood to signify a definite population of hydrogen atoms attached to a skeletal structure. Acyclic parent hydrides are always saturated and unbranched, for example, pentane and trisilane. Cyclic parent hydrides are usually either fully saturated, for example, cyclopentane, cyclotrisiloxane, azepane, bicyclo[3.2.1]octane, and spiro[4.5]decane, or fully unsaturated, i.e., they contain the maximum number of noncumulative double bonds, for example benzene, pyridine, 1,3-oxazole, 1H-phenalene, phenanthroline, and benzo[a]acridine. Also, there are parent hydrides that are partially saturated, for example 1H-spiro[[1,3]-dioxolane-2,1′-indene], and there are combinations of cyclic and acyclic hydrides, having traditional retained names, for example toluene.

Names of parent hydrides that do not contain skeletal carbon atoms, for example trisilane, are not designated as preferred IUPAC names in these recommendations. Instead they are called preselected names, i.e., they are used to generate preferred IUPAC names for derivatives substituted by organic (carbon-containing) substituents (see P-12.2). They may, however, become IUPAC preferred names depending on decisions of a team formed to decide IUPAC preferred names for inorganic (noncarbon-containing) compounds.

Names of parent hydrides are either traditional names that are retained or systematic names formed in accordance with specific rules. Rules and names must be unambiguous and clear. In order to achieve this goal and keep rules simple and concise, the rules for selecting preferred IUPAC names and preselected names of parent hydrides and prefixes denoting substituent groups derived from parent hydrides are not provided in this Chapter. These rules are given in Chapter P-5.

P-21 MONONUCLEAR AND ACYCLIC POLYNUCLEAR PARENT HYDRIDES

P-21.1 Mononuclear parent hydrides
P-21.2 Acyclic polynuclear parent hydrides

P-21.1.1 Mononuclear parent hydrides with standard bonding numbers

P-21.1.1.1 Systematic names

The names of mononuclear hydrides of the elements for use as parents in naming organic compounds by substitutive nomenclature are given in Table 2.1. Most are formed systematically by citing the ‘a’ term of the element (see Table 2.4) with elision of the terminal letter ‘a’ before the ending ‘ane’ (patterned after methane), for example borane for BH₃ and silane for SiH₄, etc. There are important exceptions: oxidane for H₂O, sulfane for H₂S, selane for H₂Se, tellane for H₂Te, polane for H₂Po, and bismuthane for BiH₃ (see Table 2.1). The systematically formed names oxane, thiane, selenane, tellurane, polonane, and bismane are Hantzsch-Widman names designating the corresponding saturated six-membered rings with a single heteroatom. The name ‘carbane’ has never been used in place of methane; it is not recommended.

P-21.1.1.2 Retained names

The common name methane is retained. The common names water, ammonia, the binary names for the hydracids of Group 17, for example hydrogen chloride, and binary names for the hydrides of Group 16, for example hydrogen sulfide, are used in these recommendations but their use as preferred IUPAC names is deferred pending publication of recommendations for the selection of preferred inorganic names; thus, no PIN label will be assigned in names including them (see P-14.8). However, systematic alternatives to these common names, e.g. oxidane for water and azane for ammonia, and for the binary names of hydracids of Group 17 and the hydrides of Group 16, for example chlorane for hydrogen chloride and sulfane for hydrogen sulfide, are necessary for naming some derivatives and for generating names of radicals, ions, and polynuclear homologues.

The names ‘phosphine’ for PH₃, ‘arsine’ for AsH₃, ‘stibine’ for SbH₃, and ‘bismuthine’ for BiH₃, are retained for use in general nomenclature.

P-21.1.2 Mononuclear parent hydrides with nonstandard bonding numbers

P-21.1.2.1 Systematic and traditional names

If the bonding number of the element differs from the standard one as defined in P-14.1 and exemplified in Table 2.1, the name of the hydride is modified by affixing the symbol λⁿ, where ‘n’ is the bonding number, to the name of the hydride (ref. 13).

The names ‘phosphorane’ for PH₅, ‘arsorane’ for AsH₅, and ‘stiborane’ for SbH₅, are retained for use in general nomenclature. However, the names ‘sulfurane’ for SH₄, ‘selenurane’ for SeH₄, ‘iodinane’ for IH₃, ‘persulfurane’ for SH₆, and ‘periodinane’ for IH₅, which have been used in recent literature, are not recommended.

Examples:

P-21.2.1 Hydrocarbons
P-21.2.2 Homogeneous acyclic parent hydrides other than hydrocarbons and boron hydrides
P-21.2.3 Heterogenous acyclic parent hydrides
P-21.2.4 Acyclic parent hydrides containing heteroatoms with nonstandard bonding numbers

The saturated unbranched acyclic hydrocarbons C₂H₆, C₃H₈, and C₄H₁₀ have the retained names ethane, propane, and butane, respectively. Systematic names for the higher members of this series consist of a numerical term (see Table 1.4) followed by the ending ‘ane’ with elision of the terminal letter ‘a’ from the numerical term. The generic name for saturated acyclic hydrocarbons (branched or unbranched) is ‘alkane’. The chain is numbered from one end to the other with arabic numbers. Brackets are employed in formulas to indicate repetition of groups in chains. For unsaturated acyclic hydrocarbons see P-31.1.

Examples:

heptane (PIN)

icosane (PIN)
(the Beilstein and CAS index name is ‘eicosane’)

tricosane (PIN)

heptacontane (PIN)

A compound consisting of an unbranched chain containing two or more identical heteroatoms saturated with hydrogen atoms is named by citing the appropriate multiplying prefix from Table 1.4 (with no elision of the terminal vowel of the multiplying prefix) followed by the name of the appropriate hydride according to P-21.1. These names are preselected (see P-12.2). Potential functionality of terminal groups, such as –SH or –OH, is ignored.

Examples:

hydrazine (retained, preselected name, see P-12.3)
diazane

nonaazane (preselected name, see P-12.2)

pentasilane (preselected name, see P-12.2)

pentaphosphane (preselected name, see P-12.2)

HS-S-S-SH
tetrasulfane (preselected name)
(not disulfanedithiol nor trisulfanethiol, see P-58.3.1)

P-21.2.3.1 Heterogeneous parent hydrides composed of alternating atoms
P-21.2.3.2 Heterogeneous parent hydrides formed by skeletal replacement (‘a’) nomenclature

P-21.2.3.1 Heterogeneous parent hydrides composed of alternating heteroatoms, i.e., [a(ba)_n hydrides], excluding carbon atoms and the halogen atoms

Compounds containing an unbranched chain of alternating atoms terminated by two identical atoms of the element coming later in the seniority order O > S > Se > Te > N > P >As > Sb > Bi > Si > Ge > Sn > Pb > B >Al > Ga > In > Tl maybe named by citing successively a multiplying prefix denoting the number of atoms of the terminal element followed by the ‘a’ term for that element, then the ‘a’ term of the other element in the chain and the ending ‘ane’. The terminal letter ‘a’ of an ‘a’ term is elided when followed by a vowel; the terminal vowel of a numerical prefix is not elided even when the ‘a’ term begins with the same vowel. When nitrogen atoms are present, amine names (see P-62) should be used because of the higher functionality of amines. This concept does not extend to other elements. These parent hydrides are preselected parent hydrides (see P-12.2) and have priority to receive preferred IUPAC names, as long as they are used to name compounds containing carbon.

This is a change from the 1993 Guide (ref. 2) where the ‘amine’ characteristic group was not recognized.

Examples:

tetrastannoxane (preselected name, see P-12.2)

SiH₃-S-SiH₃
disilathiane (preselected name, see P-12.2)

HS-O-SH
dithioxane (preselected name, see P-12.2)

PH₂-Se-PH₂
diphosphaselenane (preselected name, see P-12.2)

SiH₃-NH-SiH₃
N-silylsilanamine (name derived from the preselected name silane, see P-12.2)
(not disilazane)

Heterogeneous acyclic parent hydrides consisting of chains containing at least one carbon atom and heteroatoms, alike or different, and terminating with C, P, As, Sb, Bi, Si, Ge, Sn, Pb, B, Al, Ga, In, or Tl are named by skeletal replacement (‘a’) nomenclature (see P-15.4.3). The use of skeletal replacement (‘a’) names as preferred IUPAC names is considered in Chapter P-5.

Examples:

2,4,7,10-tetraoxaundecane (PIN)

7,10-dioxa-2-thia-4-silaundecane (PIN)

2-oxa-7-thia-3,6-disilaoctane (PIN)

P-21.2.4.1 Heteroatoms with nonstandard bonding numbers are denoted by the λⁿ symbol which is placed after each appropriate locant (see P-14.1). Low numbering is first given to the heteroatoms in the usual manner without regard to nonstandard bonding numbers. When there is a choice, lower locants are given to a higher nonstandard bonding number.

Examples:

2λ⁴-trisulfane (preselected name, see P-12.2)

1λ⁶-trisulfane (preselected name, see P-12.2)

1λ⁵,2λ⁵,3λ⁵-triphosphane (preselected name, see P-12.2)
(not tri-λ⁵-phosphane)

1λ⁵,3λ⁵-diphosphoxane (preselected name, see P-12.2)

Examples:

2λ⁶,5λ⁴-hexasulfane (preselected name; see P-12.2)

2,5λ⁶,8λ⁴,11-tetrathiadodecane (PIN)

P-22.1 Monocyclic hydrocarbons
P-22.2 Heteromonocyclic parent hydrides

P-22.1.1 The names of saturated monocyclic hydrocarbons are formed by attaching the nondetachable prefix ‘cyclo’ to the name of the acyclic saturated unbranched hydrocarbon with the same number of carbon atoms. The generic name of monocyclic hydrocarbons is ‘cycloalkane’. Numbering proceeds sequentially around the ring, usually clockwise. For unsaturated monocyclic hydrocarbons, see P-31.1.

Examples:

cyclopropane (PIN)

cyclohexane (PIN)

cyclotetradecane (PIN)

Preferred IUPAC names for fully unsaturated monocyclic hydrocarbons are those of the corresponding cycloalkapolyenes (see P-31.1.3.1).

Benzene is the retained name for C₆H₆; the name [6]annulene is not recommended.

In the numbering of annulenes, the locant ‘1’ is placed at any carbon atom in structures having an even number of carbon atoms; in annulenes having an odd number of carbon atoms, the locant ‘1’ is assigned to the carbon atom bearing the indicated hydrogen (see P-14.7). In cycloalkapolyene structures, the locant ‘1’ is always assigned to a carbon atom of a double bond.

Examples:

benzene (PIN; retained name)
(not [6]annulene)

[10]annulene
cyclodeca-1,3,5,7,9-pentaene (PIN, P-25.3.2.1.1)

[12]annulene
cyclododeca-1,3,5,7,9,11-hexaene (PIN, P-31.1.3.1)

(I)	(II)
(I) 1H-[7]annulene (II) cyclohepta-1,3,5-triene (PIN, P-31.1.3.1)
(I)	(II)
(I) 1H-[13]annulene (II) cyclotrideca-1,3,5,7,9,11-hexaene (PIN, P-31.1.3.1)

Toluene, xylene, and mesitylene are specific parent hydrides that are composed of two components, one cyclic and the other acyclic and saturated. These names are retained due to a long and well established tradition. Toluene and xylene are preferred IUPAC names, but are not freely substitutable; toluene is substitutable under certain conditions, but only for general nomenclature (see P-15.1.8 for a general substitution rules for retained names).

Mesitylene is a retained name in general nomenclature only and cannot be substituted.

The names cumene and cymene are not retained.

In the 1993 Guide (ref. 2), these parent hydrides were retained but only limited substitution was allowed.

	toluene (PIN) (no substitution; for general nomenclature substitution is permitted only by groups listed in P-15.1.8.2.2)
	xylene (1,2-, 1,3-, and 1,4-isomers, PINs) (no substitution) o-, m-, and p-xylene
	mesitylene (no substitution) 1,3,5-trimethylbenzene (PIN)

P-22.2.1 Retained names of heteromonocycles
P-22.2.2 Heteromonocyclic parent hydrides with 3-10 membered rings (Hantzsch-Widman names)
P-22.2.3 Heteromonocyclic parent hydrides named by skeletal replacement (‘a’) nomenclature
P-22.2.4 Heteromonocycles with eleven or more members with the maximum number of noncumulative double bonds
P-22.2.5 Homogeneous monocyclic parent hydrides other than boron hydrides
P-22.2.6 Heteromonocyclic parent hydrides composed of repeating heterounits
P-22.2.7 Heteromonocyclic parent hydrides having heteroatoms with nonstandard bonding numbers

Retained names for ‘mancude’ heteromonocycles and their chalcogen analogues are listed in Table 2.2.

Note: The term ‘mancude’ is an acronym for MAximum number of NonCUmulative DoublE bonds.

Retained names for saturated heteromonocycles are given in Table 2.3.

Heteromonocyclic parent hydrides with no more than ten ring members and containing one or more heteroatoms are named by using the extended Hantzsch-Widman system (ref. 21), a modification of the method published in the 1979 recommendations (ref. 1) that removes the need for many of the explanatory footnotes and comments. Homogeneous heteromonocyclic ring names are preselected (see P-12.2). The recommendations in the 1979 recommendations continue to be used in CAS fused ring names.

The elements aluminium, gallium, indium, and thallium are now included in the recommended Hantzsch-Widman system and mercury has been deleted.

P-22.2.2.1 Constructing and numbering Hantzsch-Widman names

P-22.2.2.1.1 Hantzsch-Widman names are formed by combining skeletal replacement (‘a’) prefix(es) for the heteroatom(s) (Table 2.4) with a stem indicating the size of the ring and the degree of hydrogenation (Table 2.5). The final ‘a’ vowel between ‘a’ prefixes and between the ‘a’ prefix and the stem are elided. Unsaturated compounds are those having the maximum number of noncumulative double bonds (mancude compounds) and at least one double bond. The presence of a single heteroatom determines the numbering in a monocyclic compound; the heteroatom has the locant ‘1’ and the numbering usually proceeds clockwise, when unsubstituted.

Hantzsch-Widman names, except for azine and oxine, are preferred IUPAC names for both the unsaturated and saturated compounds. Azine must not be used for pyridine because of its long-established use as a class name for =N-N= compounds (see P-68.3.1.2.3). Oxine must not be used for pyran because it has been used as a trivial name for quinolin-8-ol.

Hantzsch-Widman names are preselected names for homogeneous heteromonocycles other than hydrocarbons (see P-22.2.5).

The final ‘e’ in Hantzsch-Widman names is required in preferred IUPAC names; it is still optional in general nomenclature. In the 1979 Rules (ref. 1), the final ‘e’ of a Hantzsch-Widman name was omitted when there was no nitrogen in the ring; in the 1993 Guide (ref. 2) this omission was made optional.

Examples:

thiepine (PIN)

oxocane (PIN)

Table 2.4 Hantzsch-Widman system prefixes (in decreasing order of seniority)

Element	Bonding Number (Valence)	Prefix		Element	Bonding Number (Valence)	Prefix
fluorine	1	fluora		antimony	3	stiba
chlorine	1	chlora		bismuth	3	bisma
bromine	1	broma		silicon	4	sila
iodine	1	ioda		germanium	4	germa
oxygen	2	oxa		tin	4	stanna
sulfur	2	thia		lead	4	plumba
selenium	2	selena		boron	3	bora
tellurium	2	tellura		aluminium	3	aluma1 (not alumina)
nitrogen	3	aza		gallium	3	galla
phosphorus	3	phospha		indium	3	indiga1 (not inda)
arsenic	3	arsa		thallium	3	thalla

¹ Compare with Table 1.5

Table 2.5 Hantzsch-Widman system stems

Ring Size	Unsaturated	Saturated		Ring Size	Unsaturated	Saturated
3	irene/irine1	irane/iridine2		7	epine	epane
4	ete	etane/etidine2		8	ocine	ocane
5	ole	olane/olidine2		9	onine	onane
6A (O, S ,Se, Te, Bi)	ine	ane		10	ecine	ecane
6B (N, Si, Ge, Sn, Pb)	ine	inane
6C (F, Cl, Br, I, P, As, Sb, B, Al, Ga, In, Tl)	inine	inane
1 See P-22.2.2.1.5.1
2 See P-22.2.2.1.5.2

P-22.2.2.1.2 A multiplicity of the same heteroatom is indicated by a multiplying prefix ‘di’, ‘tri’, ‘tetra’, etc., placed before the appropriate ‘a’ term. The final letter ‘a’ of a multiplying prefix is elided before a vowel, e.g., tetrazole, not tetraazole. Lowest possible locants are assigned to heteroatoms, locant ‘1’ being assigned to one of the heteroatoms. Locants are cited at the front of the name, i.e., before the skeletal replacement (‘a’) term and any preceding numerical prefixes.

Examples:

1,5-diazocine (PIN)

1,3-dioxolane (PIN)

Examples:

1,3-thiazole (PIN)

1,2-oxathiolane (PIN)

1,2,6-oxadithiepane (PIN)
(not 1,3,7-oxadithiepane;
the locant set ‘1,2,6’ is lower than ‘1,3,7’)

1,6,2-dioxazepane (PIN)
(not 1,3,4-dioxazepane;
not 1,3,7-dioxazepane;
not 1,6,5-dioxazepane;
the locant set ‘1,2,6’ is lower than ‘1,3,4’, ‘1,3,7’, or ‘1,5,6’)

Example:

1,2,5-oxazaphosphole (PIN)
(not 1,5,2-oxazaphosphole;
N has priority over P for lowest locant)

After the maximum number of noncumulative double bonds has been assigned to the ring structure, any ring atom with a bonding number of three or higher connected to adjacent ring atoms by single bonds only, and carrying one or more hydrogen atoms, is designated by indicated hydrogen (see P-14.7). If there is a choice, such ring atoms are assigned low locants.

Examples:

1H-azepine (PIN)

1H-phosphole (PIN)

2H-1,3-dioxole (PIN)

As shown in Table 2.5, for mancude three-membered rings and saturated three-, four-, and five-membered rings two stems are recommended. They are used as follows to generate preferred IUPAC names:

P-22.2.2.1.5.1 The stem ‘irine’ is used in place of ‘irene’ for rings only containing nitrogen heteroatoms; otherwise the stem ‘irene’ is used.

Examples:

1H-azirine (PIN)

oxirene (PIN)

oxazirene (PIN)

Examples:

oxetane (PIN)

azetidine (PIN)

1,2-oxaphospholane (PIN)

1,2,3-oxathiazolidine (PIN)

The stem for six-membered rings depends on the least senior heteroatom in the ring, i.e., the heteroatom whose name directly precedes the stem. Heteroatoms are divided into three groups, A, B, and C, each corresponding to a stem for the unsaturated and for the saturated compound (Table 2.5). The stem is selected in accordance with the group to which the least senior heteroatom belongs.

Examples:

1,4-dioxine (PIN)

1,3-thiaselenane (PIN)

1,3,5-triazine (PIN)

1,3-oxazinane (PIN)

1,3,5-triphosphinine (PIN)

1,3-oxarsinane (PIN)

hexasilinane (preselected name; see P-12.2)

1,3,5,2,4,6-triphosphatriborinine
( preselected name; see P-12.2)

1,3,5,2,4,6-triphosphatriborinane (preselected name; see P-12.2)

All locants are omitted for parent Hantzsch-Widman names if there is only one heteroatom or if there is no ambiguity if locants are omitted.

Examples:

thiepine
(not 1-thiepine)

   
1H-tetrazole
(not 1H-1,2,3,4-tetrazole)

Mancude and saturated heteromonocyclic compounds with up to and including ten ring members are named by the extended Hantzsch-Widman system (see P-22.2.2). For monocyclic rings with eleven and more ring members, skeletal replacement (‘a’) nomenclature (see P-15.4) is used for the fully saturated or fully unsaturated compounds ([n]annulenes).

P-22.2.3.1 Skeletal replacement (‘a’) names are formed by placing skeletal replacement (‘a’) prefixes (see Table 2.4) in front of the name of the corresponding cycloalkane or annulene, and, when more than one heteroatom is present, in the following decreasing seniority order: F > Cl > Br > I > O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl. For numbering, see P-22.2.3.2. The selection of preferred IUPAC names is discussed in P-52.2.3.

Examples:

thiacyclododecane (PIN)

1-azacyclotetradeca-1,3,5,7,9,11,13-heptaene (PIN)
aza[14]annulene

P-22.2.3.2.1 When a single heteroatom is present in the ring, it is assigned the locant ‘1’, which is omitted in the name, unless a locant for an indicated hydrogen atom is present. Numbering usually proceeds clockwise, unless substituted. Low locants are assigned first to the heteroatom and then to unsaturated sites (see P-31.1.3.2). When required, locants for indicated hydrogen atoms are assigned in accordance with P-14.7

. Examples:

1-oxacycloundeca-2,4,6,8,10-pentaene (PIN)
oxa[11]annulene

1-azacyclotrideca-2,4,6,8,10,12-hexaene (PIN)
1H-1-aza[13]annulene

Examples:

1,5-dithiacyclododecane (PIN)
(not 1,9-dithiacyclododecane)

1,2,5,8-tetrasilacyclotridecane (PIN)
(other locant sets are: ‘1,2,8,11’, ‘1,4,7,8’, ‘1,4,5,11’, and ‘1,4,10,11’,
none of which are lower than ‘1,2,5,8’)

Examples:

1-thia-5-selenacyclododecane (PIN)

1-oxa-2-azacyclododeca-3,5,7,9,11-pentaene (PIN)
2H-1-oxa-2-aza[12]annulene

1-oxa-2-aza-11-silacyclotetradecane (PIN)

1-oxa-4-selena-11-azacyclotrideca-2,5,7,9,12-pentaene (PIN)
11H-1-oxa-4-selena-11-aza[13]annulene

Names of heteromonocyclic parent components with more than ten ring members are discussed in this subsection. They are used to generate names of preferred IUPAC names of fused ring systems; they have also been used as names for the heteromonocycle in general nomenclature as an alternative to cycloalkapolyene names that are preferred IUPAC names for the heteromonocycles.

A heteromonocyclic parent component having more than ten ring members and the maximum number of noncumulative double bonds (mancude) is named by changing the ending ‘ane’ of the name corresponding to the saturated heteromonocycle (see P-22.2.3) to ‘ine’. Locants are cited at the front of the name, in the order of citation of the corresponding skeletal replacement (‘a’) prefixes. Indicated hydrogen atoms are designated as required. In organometallic nomenclature, a modified skeletal replacement (‘a’) nomenclature is allowed in smaller rings to create metallacycle parent hydrides (see P-69.4).

For examples of fusion compounds including this type of heteromonocyclic component, see P-25.2.2.4, P-25.3.6.1 and P-25.3.7.1.

Examples:

1,8-dioxacyclooctadecane (PIN)

1,8-dioxacyclooctadecine
1,8-dioxacyclooctadeca-2,4,6,9,11,13,15,17-octaene (PIN)

1-oxa-4,8,11-triazacyclotetradecane (PIN)

2H-1,4,8,11-oxatriazacyclotetradecine
1-oxa-4,8,11-triazacyclotetradeca-3,5,7,9,11,13- hexaene (PIN)

A saturated heteromonocycle consisting of identical heteroatoms can be named by adding the prefix ‘cyclo’ to the name of the saturated unbranched chain that has the same number of identical atoms. For alternative methods, see the Hantzsch-Widman extended system described in P-22.2.2 for rings with 3 through 10 members and skeletal replacement (‘a’) nomenclature described in P-22.2.3. For the names that are preselected (see P-12.2) but used to generate preferred IUPAC names for organic derivatives, see P-52.1.5.

Examples:

cyclopentaazane
pentazolidine
(preselected name; see P-22.2.2.1.5)

cyclohexagermane
hexagerminane
(preselected name; see P-12.2)

cyclododecasilane
dodecasilacyclododecane
(preselected name; see P-12.2)

Names may be constructed by citing successively the prefix ‘cyclo’, a multiplying affix (‘di’, ‘tri’, ‘tetra’, etc.) indicating the number of each element in the ring, the skeletal replacement (‘a’) terms for the atoms of the repeated unit first cited in the order Tl > In > Ga > Al > Pb > Sn > Ge > Si > Bi > Sb> As > P > N > Te > Se > S> O, and the ending ‘ane’. The terminal letter of a skeletal replacement (‘a’) term is elided when followed by a vowel; the terminal letter of a multiplying affix is not elided even when the ‘a’ term begins with a vowel. Numbering starts at one of the skeletal atoms of the element cited last in the name and proceeds continuously around the ring. For alternative methods, see P-22.2.2 for Hantzsch-Widman names and P-22.2.3 for monocycles with more than ten ring members. For the preselected names (see P-12.2) used to generate the preferred IUPAC names for organic derivatives of these heteromonocyclic hydrides, see P-52.1.5.2.

Examples:

cyclotetragermoxane
1,3,5,7,2,4,6,8-tetroxatetragermocane
(preselected name, see P-12.2)

cyclotriboraphosphane
1,3,5,2,4,6-triphosphatriborinane
(preselected name, see P-12.2)

cycloheptasiloxane
1,3,5,7,9,11,13-heptaoxa-2,4,6,8,10,12,14-heptasilacyclotetradecane
(preselected name, see P-12.2, P-22.2.3.2.3)

P-22.2.7.1 The λ-convention is used to denote heteroatoms with nonstandard bonding numbers in heteromonocycles (see P-14.1). The symbol λⁿ, where n is the bonding number, is cited immediately after the locant denoting the heteroatom with the nonstandard bonding number. The indicated hydrogen symbol H, if required to denote saturated skeletal atoms, is cited at the front of the complete name with the appropriate locant(s).

Examples:

1H-1λ⁴-thiophene (PIN)

3H-1λ⁴-thiophene (PIN)

1λ³-iodinane (PIN)

1H-1λ⁵-iodole (PIN)

1,3λ⁵-oxaphosphole (PIN)

1λ⁴,3-thiazine (PIN)

1-oxa-4λ⁴-thiacyclotetradecane (PIN)

1-oxa-4,8λ⁴-dithiacyclododecane (PIN)

1H-1λ⁴-thiepine (PIN)

1H-1λ⁴,3λ⁴-dithiepine (PIN)
[the (–SH₂–) group, which includes an indicated hydrogen, is preferred for the lowest locant]

1λ⁶-thiopyran (PIN)
(this heteromonocycle has the maximum number of double bonds and one double bond at every position;
hence, no indicated hydrogen is cited for the sulfur atom)

Examples:

1λ⁴,3-dithiole (PIN)

1-oxa-4λ⁴,12-dithiacyclotetradecane (PIN)

P-23.0 Introduction
P-23.1 Definitions and terminology
P-23.2 Naming and numbering of von Baeyer hydrocarbons
P-23.3 Heterogeneous heterocyclic von Baeyer parent hydrides
P-23.4 Homogeneous heterocyclic von Baeyer parent hydrides
P-23.5 Heterogeneous heterocyclic von Baeyer parent hydrides composed of alternating heteroatoms
P-23.6 Heterocyclic polyalicyclic parent hydrides having heteroatoms with nonstandard bonding numbers
P-23.7 Retained names for von Baeyer parent hydrides

This section is based on the recent publication “Extension and Revision of the von Baeyer system for naming polycyclic compounds (including bicyclic compounds) (IUPAC Recommendations 1999)” (ref. 7). It supersedes Rules A-31, A-32, and B-14 in the 1979 Recommendations (ref. 1) and Rule R-2.4.2 in the 1993 Recommendations (ref. 2). No modifications to the 1999 publication have been made in this Section.

This Section deals only with saturated polyalicyclic ring systems named by the von Baeyer system; for unsaturated systems, see Section P-31.1.4. For naming substituent groups derived from saturated polyalicyclic ring systems, see Section P-29.

P-23.1 DEFINITIONS AND TERMINOLOGY

P-23.1.1 A ‘bridgehead’ is any skeletal atom of the ring system that is bonded to three or more skeletal atoms (excluding hydrogen).

P-23.1.2 A ‘bridge’ is an unbranched chain of atoms or an atom or a valence bond connecting two bridgeheads.

P-23.1.3 The ‘main ring’ is the ring system that includes as many skeletal atoms of the polycyclic system as possible.

P-23.1.4 The ‘main bridge’ is the bridge included in a bicyclic system and the first selected bridge in a polycyclic system.

P-23.1.5 Two bridgeheads are selected as ‘main bridgeheads’. These two bridgeheads are included in the main ring and connected by the main bridge.

P-23.1.6 A ‘secondary bridge’ is any bridge not included in the main ring or the main bridge.

P-23.1.7 An ‘independent secondary bridge’ links bridgeheads which are part of the main ring or main bridge.

P-23.1.8 A ‘dependent secondary bridge’ links at least one bridgehead that is part of a secondary bridge.

P-23.1.9 A ‘polycyclic system’ contains a number of rings equal to the minimum number of scissions required to convert the system into an acyclic skeleton. The number of rings is indicated by the nondetachable prefix ‘bicyclo’ (not dicyclo), ‘tricyclo’, ‘tetracyclo’, etc.

P-23.2 NAMING AND NUMBERING OF VON BAEYER HYDROCARBONS

Bi- and polycyclic hydrocarbons that are treated by the von Baeyer system are named by the following rules applied in order until a decision is reached.

P-23.2.1 Selection of the main ring
P-23.2.2 Naming bicyclic alicyclic hydrocarbons
P-23.2.3 Numbering bicyclic alicyclic hydrocarbons
P-23.2.4 Selection of the main bridge
P-23.2.5 Naming and numbering tricyclic alicyclic hydrocarbons
P-23.2.6 Naming and numbering polyalicyclic hydrocarbons

The main ring of a polycyclic hydrocarbon ring system is selected so as to include as many skeletal atoms of the structure as possible. The main ring is shown as bold lines in subsections P-23.2.1 through P-23.2.6.

Examples:

a six-membered main ring

a seven-membered main ring

Saturated homogeneous bicyclic hydrocarbons having two or more atoms in common are named by prefixing ‘bicyclo’ to the name of the acyclic hydrocarbon having the same total number of skeletal atoms. The numbers of skeletal atoms in each of the two segments connecting the main bridgeheads and in the main bridge are given by arabic numbers cited in descending numerical order, separated by full stops, and enclosed in square brackets.

Example:

bicyclo[3.2.1]octane (PIN)

The bicyclic ring system is numbered starting with one of the bridgeheads and proceeding first along the longer segment of the main ring to the second bridgehead, then back to the first bridgehead along the unnumbered segment of the main ring. Numbering is completed by numbering the main bridge beginning with the atom next to the first bridgehead.

Examples:

bicyclo[3.2.1]octane (PIN)

bicyclo[4.4.2]dodecane (PIN)

In a polycyclic ring system there is more than one bridge connecting atoms of the main ring and/or the main bridge. In the tricyclic ring below, the main ring is shown in solid bold lines, and the main bridge, shown in the dashed lines, is the bridge that includes as many of the atoms as possible that are not included in the main ring. Bridges other than the main bridge are called ‘secondary bridges’, appearing below as a normal solid line.

Example:

P-23.2.5.1 Tricyclic hydrocarbons having an independent secondary bridge are named on the basis of a bicyclic system described in P-23.2.2. Rings not described by the bicyclic system are defined by citing the number of atoms in the independent secondary bridge as an arabic number. The locants of the two attachment points of the independent secondary bridge to the main ring are cited as a pair of superscript arabic numbers (lower number is cited first) separated by a comma.

The name of the tricyclic system is then constructed by citing:

(a) the prefix ‘tricyclo’, in place of ‘bicyclo’, indicating the presence of three rings in the polyalicyclic system;

(b) numbers indicating the bridge lengths, starting with the two branches of the main ring (shown as a bold line in the structures below), followed by the main bridge (shown in dotted bold lines in the structures below), and the secondary bridge (with superscript locants separated by commas indicating its points of attachment to the main ring), all separated by full stops and placed in brackets, for example [2.2.1.0^2,6];

(c) the name of the acyclic hydrocarbon having the same total number of skeletal atoms.

tricyclo[2.2.1.0^2,6]heptane (PIN)

After the main ring and main bridge have been numbered, the independent secondary bridge is numbered continuing from the higher numbered bridgehead of the main ring.

Examples:

tricyclo[9.3.3.1^1,11]octadecane (PIN)

tricyclo[4.2.2.2^2,5]dodecane (PIN)
[the secondary bridge is numbered starting from bridgehead 5, the higher (than 2) numbered bridgehead]

Polycyclic analogues of saturated bi- and tricyclic ring systems (P-23.2.3 and P-23.2.5) are named as described in the following subsections. Independent and dependent secondary bridges are considered here. Rules for numbering all secondary bridges and for naming all polyalicyclic systems are described; their application follows those described for naming and numbering bicyclic systems as described in P-23.2.1 through P-23.2.4 above. An additional rule is necessary to select the main bridge and the secondary bridges.

P-23.2.6.1 Naming polycyclic alicyclic hydrocarbons

Rings not designated by the bicyclic system described above (P-23.2.2) are defined by citing the number of atoms in each secondary bridge as an arabic number. The locants of the two attachment points of each secondary bridge to the main ring are cited as a pair of superscript arabic numbers (lower first) separated by a comma. The numbers indicating independent secondary bridges (bridges that connect atoms of the bicyclic system) are cited in decreasing order. The procedure for construction of names is given in the following subsections.

P-23.2.6.1.1 The prefixes ‘tricyclo’, ‘tetracyclo’, etc., in place of vbicyclo’, indicate the number of rings in the polyalicyclic system. The number of rings is equal to the number of bond cuts necessary to transform the polycyclic system into an acyclic skeleton, unbranched or branched.

P-23.2.6.1.2 The number of atoms in each bridge additional to the main bridge, i.e., the secondary bridges, is indicated by arabic numbers separated by full stops and cited in decreasing numerical order following those describing the bicyclic system, except as provided by P-23.2.6.1.3. The location of each secondary bridge is indicated by the arabic number locants of the bicyclic structure already numbered; these locants are cited as superscripts to the arabic number denoting its length (number of atoms) and separated by a comma. The assemblage of arabic numbers denoting the length of bridges with superscript numbers, if necessary, is commonly called the ‘von Baeyer descriptor’ and is enclosed in brackets.

P-23.2.6.1.3 Independent secondary bridges are cited before dependent secondary bridges. The numbers indicating dependent secondary bridges are cited in decreasing order (the third example in P-23.2.6.3 illustrates this order of citation).

P-23.2.6.1.4 The name is terminated by the name of the alkane representing the total number of ring atoms; this number corresponds to the sum of the arabic numbers in the numerical descriptor enclosed by brackets plus two (for the two main bridgehead atoms); for example, in the name ‘bicyclo[2.2.1.0^2,6]heptane’ for the following structure, the total number of ring atoms, 7, equals the sum of the primary numbers in the descriptor, [2 + 2 + 1+ 0], + 2.

P-23.2.6.2 Selection of the main bridge and secondary bridges

There are often a number of choices to be made in the selection of the main bridge and the secondary bridges. To make such choices, the following criteria are applied in order until a decision can be made.

Note: Numberings shown in the examples below follow the rules given in P-23.2.6.3.

Example:

	not
tricyclo[4.3.1.12,5]undecane (PIN) correct		tricyclo[5.2.1.12,6]undecane incorrect

P-23.2.6.2.2 If there is a choice for independent secondary bridges, the first cited must be as long as possible. Then, if relevant, the second must be as long as possible, etc.

Example:

	not
tetracyclo[8.6.6.52,9.123,26]octacosane (PIN) correct		tetracyclo[8.6.6.42,9.223,25]octacosane incorrect

P-23.2.6.2.3 The number of dependent secondary bridges is to be kept to a minimum.

Example:

	not
tetracyclo[5.3.2.12,4.03,6]tridecane (PIN) correct		tetracyclo[5.3.2.12,4.06,13]tridecane incorrect

P-23.2.6.2.4 The superscript locants for the secondary bridges must be as low as possible when considered as a set in ascending numerical order, the decision being made at the first point of difference.

Examples:

	not
tetracyclo[5.3.2.12,4.03,6]tridecane (PIN) correct		tetracyclo[5.3.2.14,6.02,5]tridecane incorrect

	not
tricyclo[5.5.1.03,11]tridecane correct		tricyclo[5.5.1.05,9]tridecane incorrect

	not
tricyclo[4.4.1.11,5]dodecane (PIN) correct		tricyclo[4.4.1.11,7]dodecane incorrect

P-23.2.6.2.5 The superscript locants shall be as low as possible when considered in the sequence of their order of citation in the name.

Examples:

	not
tetracyclo[5.5.2.22,6.18,12]heptadecane (PIN) correct		tetracyclo[5.5.2.28,12.12,6]heptadecane incorrect

	not
pentacyclo[3.3.0.02,4.03,7.06,8]octane (PIN) correct		pentacyclo[3.3.0.02,8.03,7.04,6]octane incorrect

P-23.2.6.3 Numbering of secondary bridges

After numbering the main ring and main bridge, independent secondary bridges are numbered before dependent secondary bridges; the numbering continues from the highest number of the main ring and main bridge. Each secondary bridge is numbered in turn starting with the independent secondary bridge linked to the highest numbered bridgehead atom of the main ring, then the independent secondary bridge linked to the next highest bridgehead atom, and so on. Each atom of a secondary bridge is numbered starting with the atom next to the higher numbered bridgehead.

Note: Rule A-32.23 in the 1979 Recommendations (ref. 1) and Rule R-2.4.2.2 in the l993 Recommendations (ref. 2) have been replaced by this Rule.

	not
tetracyclo[4.4.2.22,5.27,10]hexadecane (PIN)

	not
tetracyclo[5.4.2.22,6.18,11hexadecane (PIN) (the first secondary bridged to be numbered is linked to bridgehead 11)

hexacyclo[15.3.2.2^3,7.1^2,12.0^13,21.0^11,25]pentacosane
(the dependent secondary bridge between atoms numbered 11 and 25 is cited last;
in the numeric descriptor, the numbers ‘15, 3, and 2’ describe the basic bicyclic system;
the numbers 2^3,7,1^2,12, and 0^13,21 correspond to the three independent secondary bridges;
the number 0^11,25 corresponds to the dependent secondary bridge)

octadecahydro-6,13-methano-2,14:7,9-dipropanodicyclohepta[a,e][8]annulene (PIN) (see P-25.4.3.4.2)
[not octadecahydro-7,14-methano-4,6:8,10-dipropanodicyclohepta[a,d][8]annulene;
in the correct name a greater number of rings of the fusion ring system to be bridged are in a horizontal row,
3 versus 2; see P-25.3.2.3.3 (a) and P-44.2.2.2.3 (b)]

	not
(I) correct		(II) incorrect
(I) pentacyclo[13.7.4.33,8.018,20.113,28]triacontane (PIN) (II) pentacyclo[13.7.4.33,13.018,20.18,28]triacontane

Explanation: In the correct name (I), the PIN, the independent bridge is numbered before the dependent bridge; locant sequence ‘3,8,18,20,13,28’ is lower than ‘3,13,18,20,8,28’ (see P-14.3.5).

P-23.2.6.4 If there is a further choice for numbering the secondary bridges, the following criteria are considered in order until a decision can be made.

P-23.2.6.4.1 Lower locants are used for the atoms of the bridge linked to the higher numbered bridgehead.

Examples:

tetracyclo[6.3.3.2^3,6.1^2,6]heptadecane (PIN)
(locants 15 and 16 are assigned to bridge linked to bridgehead atom 3 not 2)

tetracyclo[6.3.3.2^2,6.1^3,6]heptadecane (PIN)
(the locant 15 is assigned to the bridge linked to bridgehead atom 3 not 2)

Example:

tetracyclo[7.4.3.2^3,7.1^3,7]nonadecane (PIN)
(the two-atom bridge between bridgehead atoms 3 and 7 is numbered before the one-atom bridge between the same two bridgehead atoms).

P-23.3.1 The only general method to name heterogeneous heterocyclic von Baeyer systems is skeletal replacement (‘a’) nomenclature. The replacement (‘a’) prefixes denoting the heteroatoms are placed in front of the name of the corresponding hydrocarbon named according to P-23.2, cited in the order: F > Cl > Br > I > O > S > Se > Te > N > P > As > Sb > Bi >Si > Ge > Sn > Pb > B > Al > Ga > In > Tl. Numbering is determined first by the fixed numbering of the hydrocarbon system.

Examples:

3-oxabicyclo[3.2.1]octane (PIN)

2-selenabicyclo[2.2.1]heptane (PIN)

P-23.3.2.1 Low locants are assigned to the heteroatoms considered together as a set compared in increasing numerical order. The preferred numbering is the lowest set at the first point of difference.

Example:

	not
3,6,8-trioxabicyclo[3.2.2]nonane (PIN) correct		3,7,9-trioxabicyclo[3.2.2]nonane incorrect

P-23.3.2.2 If there is still a choice, low locants are assigned in accord with the decreasing seniority order of heteroatoms O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl.

Example:

2-oxa-4-thiabicyclo[3.2.1]octane (PIN)

Heterocyclic von Baeyer systems composed entirely of the same heteroatom are named:

(a) as described for bi- and polycyclic hydrocarbons in P-23.2 using the name of the acyclic parent hydride that has the same total number of skeletal atoms; or

(b) by skeletal replacement nomenclature using the replacement (‘a’) prefixes described in P-22.2.5, in which the total number of heteroatoms is indicated by a numerical term.

Examples:

(a) bicyclo[4.2.1]nonasilane (preselected name; see P-12.2)
(b) nonasilabicyclo[4.2.1]nonane

(a) tricyclo[5.3.1.1^2,6]dodecasilane (preselected name; see P-12.2)
(b) dodecasilatricyclo[5.3.1.1^2,6]dodecane

P-23.5.1 Heterogeneous von Baeyer systems composed of alternating skeletal heteroatoms may be named in two ways:

(a) by citing a nondetachable prefix ‘bicyclo’, ‘tricyclo’, etc. before a von Baeyer descriptor (see P-23.2.2, P-23.2.5, and P-23.2.6) enclosed in square brackets and then, successively:

(i) a multiplying prefix, ‘di’, ‘tri’, etc., denoting the number of heteroatoms that are first cited as replacement (‘a’) terms;

(ii) the sketetal replacement (‘a’) terms denoting the heteroatoms of the system in the reverse order of the recommended seniority for ‘a’ prefixes (for example, Si before O; see P-23.3.1);

(iii) the ending ‘ane’.

Numbering is the same as for the corresponding hydrocarbon (see P-23.2.3 and P-23.2.6.3);

(b) by applying normal skeletal replacement (‘a’) nomenclature to the corresponding hydrocarbon.

Examples:

(a) bicyclo[3.3.1]tetrasiloxane (preselected name; see P-12.2)
(b) 2,4,6,8,9-pentaoxa-1,3,5,7-tetrasilabicyclo[3.3.1]nonane

(a) tricyclo[5.1.1.1^3,5]tetrasilathiane (preselected name; see P-12.2)
(b) 2,4,6,8,9,10-hexathia-1,3,5,7-tetrasilatricyclo[5.1.1.1^3,5]decane

(a) tricyclo[3.3.1.1^3,7]tetrasiloxane (preselected name; see P-12.2)
(b) 2,4,6,8,9,10-hexaoxa-1,3,5,7-tetrasilatricyclo[3.3.1.1^3,7]decane

Examples:

(a) 1N-tricyclo[3.3.1.1^2,4]pentasilazane (preselected name; see P-12.2)
(b) 1,3,5,7,10-pentaaza-2,4,6,8,9-pentasilatricyclo[3.3.1.1^2,4]decane

(a) 1Si-tricyclo[3.3.1.1^2,4]pentasilazane (preselected name; see P-12.2)
(b) 2,4,6,8,9-pentaaza-1,3,5,7,10-pentasilatricyclo[3.3.1.1^2,4]decane

Compounds in which each silicon atom is linked to three oxygen atoms and in which every atom of oxygen is linked to two silicon atoms are named generically silasesquioxanes. Similarly, when the oxygen atoms are replaced by S, Se, Te, or N atoms, the compounds are generically called silasesquithianes, silasesquiazanes, and so forth. They are named by the method described in P-23.5.1 (a). Silasesquioxanes have the general formula Si_2nH_2nO_3n. The names tetrasilasesquioxanes (n = 2), hexasilasesquioxanes (n = 3), etc., are class names indicating Si_2nH_2nE_3n where E = O; and similarly when E = S, Se, Te, or N. Silasesquiazanes have the general formula Si_2nH_5nN_3n.

tetracyclo[5.5.1.1^3,11.1^5,9]hexasiloxane (a hexasilasesquioxane)
(preselected name; see P-12.2)
2,4,6,8,10,12,13,14,15-nonaoxa-1,3,5,7,9,11-hexasilatetracyclo[5.5.1.1^3,11.1^5,9]pentadecane

P-23.6.1 The λ-convention, characterized by the symbol λⁿ, where ‘n’ is the bonding number of the heteroatom, is used to identify heteroatoms with nonstandard bonding numbers (see P-14.1). The symbol is placed before the appropriate ‘a’ prefix.

Example:

3λ⁴-thiabicyclo[3.2.1]octane (PIN)

Example:

2λ⁵,3-diarsabicyclo[2.2.1]heptane (PIN)

The retained names adamantane and cubane are used in general nomenclature and as preferred IUPAC names. The name quinuclidine is retained for general nomenclature only (see Table 2.6). The name prismane is no longer recommended.

P-24.0 Introduction
P-24.1 Definitions
P-24.2 Spiro ring systems with only monocyclic ring components
P-24.3 Monospiro ring systems containing two identical polycyclic ring components
P-24.4 Three identical polycyclic ring components spirofused together
P-24.5 Monospiro ring systems with different ring components at least one of which is a polycyclic ring system
P-24.6 Unbranched polyspiro ring systems with different ring components, one being a polycyclic ring system
P-24.7 Branched polyspiro ring systems
P-24.8 Spiro ring systems containing atoms with nonstandard bonding numbers

This Section is based on the recent publication ‘Extension and Revision of the Nomenclature for Spiro Compounds, IUPAC Recommendations1999’ (ref. 8). It supersedes Rules A-41, A-43, B-10, and B-12 in the 1979 Recommendations (ref. 1) and Rule R-2.4.3 in the 1993 Recommendations (ref. 2). The alternative methods given by Rules A-42 and B-11 in the 1979 recommendations (ref. 1) have been abandoned.

Section P-24.5.2 takes into account appropriate modifications to Section SP-4.1 of the 1999 publication (ref. 8). Also in P-24.5.3, P-24.6, P-24.7 and P-24.8.4 some modifications on the usage of brackets have been made to SP-4, SP-5 and SP-7 of the 1999 publication (ref. 8).

The spiro ring systems consisting only of monocyclic rings in this Section are saturated systems; for unsaturated systems, see Section P-31.1.5.1.

For naming substituent groups see Sections P-29 and P-32.1.3.

P-24.1 DEFINITIONS

A ‘spiro union’ is a linkage between two rings that consists of a single atom common to both rings. A ‘free spiro union’ is a linkage that constitutes the only direct union between the two rings.

a free spiro union

a non-free spiro union

Spirofusion is the creation of one, and only one, common atom between two rings or ring systems, each ring or ring system contributing one, and only one, atom to the spiro ring system. It is analogous to the ortho- or ortho- and peri-fusion that creates common bonds between mancude rings or ring systems. Traditionally, ortho- or ortho- and peri-fusion has been called ‘fusion’, with no reference to its specific type of fusion. To avoid any ambiguity, the term ‘spiro’ must always be specified when added to the term ‘fusion’.

P-24.2 SPIRO RING SYSTEMS WITH ONLY MONOCYCLIC RING COMPONENTS

P-24.2.0 Introduction
P-24.2.1 Monospiro alicyclic ring systems
P-24.2.2 Linear polyspiro alicyclic ring systems
P-24.2.3 Branched polyspiro alicyclic ring systems
P-24.2.4 Heterocyclic spiro ring systems

This Section is concerned only with saturated spiro ring systems that consist only of monocyclic rings. For unsaturated alicyclic spiro ring systems, see Section P-31.1.5.

P-24.2.1 Monospiro alicyclic ring systems

Monospiro parent hydrides consisting of two saturated cycloalkane rings are named by placing the nondetachable prefix ‘spiro’ before the name of the unbranched acyclic hydrocarbon with the same total number of skeletal atoms. The number of skeletal atoms linked to the spiro atom in each ring is indicated by arabic numbers separated by a full stop, cited in ascending order and enclosed in square brackets; this descriptor (called in these recommendations the ‘von Baeyer spiro descriptor’) is placed between the spiro prefix and the name of the acyclic alkane. Numbering starts in the smaller ring, if one is smaller, at a ring atom next to the spiro atom and proceeds first around that ring, then through the spiro atom and around the second ring.

Examples:

spiro[4.5]decane (PIN)

spiro[4.4]nonane (PIN)

Polyspiro parent hydrides consisting of unbranched assemblies of three or more saturated cycloalkane rings are named using the nondetachable prefixes ‘dispiro’, ‘trispiro’, etc., according to the number of spiro atoms present, cited in front of the name of the acyclic hydrocarbon that has the same number of skeletal atoms. The von Baeyer spiro descriptor indicates the number of carbon atoms linking the spiro atoms by arabic numbers that are cited in order, starting at the smaller terminal ring if one is smaller, and proceeding consecutively, always by the shorter path, to the other terminal ring through each spiro atom and then back to the first spiro atom; the numbers are separated by full stops and enclosed in square brackets. The compound is numbered in the order in which the numbers of the von Baeyer spiro descriptor are cited, including spiro atoms when encountered for the first time. Each time a spiro atom is reached for the second time its locant, which has already been assigned, is cited as a superscript number to the number of the preceding linking atoms.

Examples:

dispiro[3.2.3⁷.2⁴]dodecane (PIN)

dispiro[2.2.3⁶.2³]undecane (PIN)

trispiro[2.2.2.2⁹.2⁶.2³]pentadecane (PIN)

P-24.2.2.1 If there is a choice of numbers for the spiro descriptor, the smaller numbers are selected because low locants must be allocated to spiro atoms.

Examples:

	not
dispiro[4.1.47.25]tridecane (PIN) correct		dispiro[4.2.48.15]tridecane incorrect

	not
dispiro[5.1.88.26]octadecane (PIN) correct		dispiro[5.2.89.16]octadecane incorrect

P-24.2.2.2 If there is still a choice of numbering, the numbers of the von Baeyer descriptors are considered in the sequence of the order of their citation. The name is selected with lower numbers at the first point of difference.

Example:

	not
trispiro[2.2.2.29.26.33]hexadecane (PIN) correct		trispiro[2.2.2.29.36.23]hexadecane incorrect

P-24.2.3 Branched polyspiro alicyclic ring systems

Branched polyspiro hydrocarbons composed only of cycloalkane rings are named using ‘dispiro’, ‘trispiro’, etc. before the name of the acyclic hydrocarbon corresponding to the total number of skeletal atoms present. The von Baeyer spiro descriptor indicates the number of skeletal atoms linking the spiro atoms by arabic numbers that are cited in order, starting at the smallest terminal ring, if one is smallest, proceeding consecutively to succeeding terminal rings through each spiro atom, always by the shortest path, and then back to the first spiro atom; the numbers are separated by full stops and enclosed in square brackets. The compound is numbered in the order in which the numbers of the spiro von Baeyer descriptor are cited, including spiro atoms when encountered for the first time. Each time a spiro atom is reached for the second time its locant is cited as a superscript number to the preceding number of linking atoms.

Examples:

trispiro[2.2.2⁶.2.2¹¹.2³]pentadecane (PIN)
(the importance of superscripts is illustrated by this example and the third example in P-24.2.2;
without the superscripts these different compounds would have identical names)

nonaspiro[2.0.0.0.2⁶.0.2⁹.0⁵.0.0.2¹³.0.2¹⁶.0¹².0⁴.0.2¹⁹.0³]henicosane (PIN)

Examples:

	not
trispiro[3.0.35.1.310.14]tetradecane (PIN) correct		trispiro[3.1.36.0.310.14]tetradecane incorrect

	not		not
tetraspiro[2.2.2.29.2.214.26.23]icosane (PIN) correct		tetraspiro[2.2.2.29.26.2.216.23]icosane incorrect		tetraspiro[2.2.26.2.2.214.211.23]icosane incorrect

P-24.2.3.2 If there is still a choice of numbering, the numbers of the von Baeyer descriptor are considered in their order of citation. The correct name has the lower numbers at first point of difference.

Example:

	not
pentaspiro[2.0.24.1.1.210.0.213.18.23]octadecane (PIN) correct		pentaspiro[2.0.24.1.1.210.0.213.28.13]octadecane incorrect

P-24.2.4 Heterocyclic spiro ring systems

P-24.2.4.1 Heterocyclic spiro ring systems named by skeletal replacement (‘a’) nomenclature
P-24.2.4.2 Homogeneous heterocyclic spiro ring systems with only monocyclic ring components
P-24.2.4.3 Heterocyclic spiro ring systems with only monocyclic ring components composed of alternating heteroatoms

P-24.2.4.1.1 When heteroatoms are present in a spiro ring system composed of only monocyclic rings, skeletal replacement (‘a’) nomenclature is used to name the heterocyclic system. The name of the corresponding hydrocarbon ring system is first constructed as described above (P-24.2.1, P-24.2.2, P-24.2.3). Heteroatoms are then introduced by using the general principles of skeletal replacement (‘a’) nomenclature. The numbering of the spiro hydrocarbon ring system is never modified by the introduction of heteroatoms, but low locants must be attributed to heteroatoms if there is a choice.

Examples:

6-oxaspiro[4.5]decane (PIN)
(not 10-oxaspiro[4.5]decane)

6,7,13,14-tetraoxadispiro[4.2.4⁸.2⁵]tetradecane (PIN)

5,6,16,17-tetraoxahexaspiro[2.0.2.0.2⁸.2.2¹³.0⁷.2⁴.0.2¹⁸.2³]docosane (PIN)

11,13,24,25-tetraoxa-12-silapentaspiro[4.0.4⁶.1.1.4¹⁴.0.4¹⁹.1¹².1⁵]pentacosane (PIN)

(a) low locants are allocated to heteroatoms as a set regardless of the kind of heteroatom;

9-oxa-6-azaspiro[4.5]decane (PIN)

(b) if there is still a choice, low locants are assigned in accord with the following decreasing seniority order of heteroatoms: F > Cl > Br > I > O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl.

Example:

7-thia-9-azaspiro[4.5]decane (PIN)

Heterocyclic spiro ring systems with only monocyclic ring components and composed entirely of the same heteroatom are named as described above using the name of the homogeneous heteroacyclic ring system that has the same total number of skeletal heteroatoms. This method is preferred to skeletal replacement (‘a’) nomenclature described in P-24.2.4.1, in which the total number of heteroatoms is indicated by a numerical term. In either method it is not necessary to give the location of the heteroatoms because all positions are modified by the same heteroatom. For the preselected names (see P-12.2) that are used to generate PIN names for organic derivatives see P-52.1.6.

Example:

spiro[4.5]decasilane (preselected name; see P-12.2)
decasilaspiro[4.5]decane

Heterocyclic spiro ring systems consisting only of monocyclic rings and having alternating skeletal heteroatoms are named by two methods.

(1) by citing a prefix such as ‘spiro’, ‘dispiro’, etc., before a von Baeyer descriptor (indicating the numbers of heteroatoms linked to each spiro atom in each ring cited in increasing order and separated by a full stop) enclosed in square brackets followed successively by:

(a) a multiplying prefix (‘di’, ‘tri’, etc) denoting the number of heteroatoms of the first cited ‘a’ term that follows;

(b) the ‘a’ terms of the hetero atoms, cited in the reverse seniority order for ‘a’ prefixes (see P-21.2.3.1), for example, Si before O;

(c) the ending ‘ane’. The hetero spiro system is numbered as is the corresponding hydrocarbon.

(2) by skeletal replacement (‘a’) nomenclature as described in P-24.2.4.1.

spiro[5.5]pentasiloxane (preselected name; see P-12.2)
1,3,5,7,9,11-hexaoxa-2,4,6,8,10-pentasilaspiro[5.5]undecane

P-24.3.1 Monospiro ring systems consisting of two identical polycyclic ring components are named by placing the nondetachable prefix ‘spirobi’ before the name of the component ring system enclosed in square brackets. The established numbering system of the polycyclic ring system component is retained with one system having primed locants. The location of the spiro atom is indicated in the name by the appropriate locants (unprimed first) placed at the front of the name.

Example:

7,7′-spirobi[bicyclo[4.1.0]heptane] (PIN)

Note: This treatment of indicated hydrogen was introduced in the 1999 publication on the nomenclature of spiro compounds (ref. 8)

Examples:

1H,1′H-2,2′-spirobi[naphthalene] (PIN)

3H,3′H-2,2′-spirobi[[1]benzothiophene] (PIN)

1,1′-spirobi[indene] (PIN)

1,1′-spirobi[isoindole] (PIN)

Examples:

1′H,2H-1,2′-spirobi[azulene] (PIN)

2′H,4H-2,4′-spirobi[[1,3]dioxolo[4,5-c]pyran] (PIN)

2′H,3H-2,3′-spirobi[[1]benzothiophene] (PIN)

Note 2: The double set of brackets in this name occurs because the spiro name requires them and brackets are used to enclose locants belonging to component names (see P-16.5.2.2).

2,4′-spirobi[[1]benzopyran] (PIN)
2,4′-spirobi[chromene]

When ring components of ‘spirobi’ compounds are named by von Baeyer nomenclature, heteroatoms are indicated by skeletal replacement (‘a’) nomenclature. The spirobi ring system is named as the saturated bi- or polycyclic alicyclic hydrocarbon and the heteroatoms are denoted by ‘a’ prefixes cited at the front of the completed ‘spirobi’ hydrocarbon name. If there is a choice, low locants are given to the spiro atom, then to the heteroatoms as described in Section P-24.2.4.1.

Note: This treatment of skeletal replacement (‘a’) prefixes was introduced in the 1999 publication on the nomenclature of spiro compounds (ref. 8).

Examples:

	not
(I)		(II)
correct		incorrect
5,6′-dithia-2,2′-spirobi[bicyclo[2.2.2]octane] (I) (PIN) [not 6,8′-dithia-2,2′-spirobi[bicyclo[2.2.2]octane] (II); the set of locants ‘5,6′ ’ in (I) is lower than ‘6,8′ ’ in (II)]

6,6′-dioxa-3,3′-spirobi[bicyclo[3.2.1]octane] (PIN)

6-sila-2,2′-spirobi[bicyclo[2.2.1]heptane] (PIN)

6-oxa-6′-thia-2,2′-spirobi[bicyclo[2.2.1]heptane] (PIN)

2-sila-2,3′-spirobi[bicyclo[3.2.1]octane] (PIN)

P-24.4.1 Dispiro ring systems with three identical polycyclic ring components are named by placing the nondetachable prefix ‘dispiroter’ before the name of the component ring system enclosed in square brackets. The multiplicative prefix ‘ter’ (see P-14.2.3) is used to indicate the repetition of identical ring components. Locants for the middle ring component are primed and for the third ring component double primed. The spiro atoms are indicated in front of the name by two pairs of locants separated by a colon. Indicated hydrogen is cited in front of these locants, if needed.

Examples:

3,3′:6′,6′′-dispiroter[bicyclo[3.1.0]hexane] (PIN)

1H,1′H,1′′H,3′H-2,2′:7′,2′′-dispiroter[naphthalene] (PIN)

Examples:

2,3′:7′,7′′-dispiroter[bicyclo[4.1.0]heptane] (PIN)

	not		nor
(I)		(II)		(III)
correct		incorrect		incorrect
1′′H,2H,5′H,7′H-1,6′:1′,2′′-dispiroter[naphthalene] (I) (PIN) [not 1′H,1′′H,2H,3′H-1,2′:5′,2′′-dispiroter[naphthalene] (II); nor 1H,2′′H,5′H,7′H-2,1′:6′,1′′-dispiroter[naphthalene] (III)

P-24.4.3 Three identical heterocyclic ring components spirofused together.

Dispiro compounds with three identical heterocyclic components may be named:

(a) by using heterocyclic monocyclic or polycyclic mancude components in the same way as for ‘spiroter’ hydrocarbons (see P-24.4.1); the numbering depends first on the fixed numbering of the heterocyclic components;

(b) by skeletal replacement (‘a’) nomenclature when the ring components are polycyclic von Baeyer ring systems; the numbering of the ‘spiroter’ von Baeyer hydrocarbon remains unchanged.

	not
correct		incorrect
1′H,2H,3′′H,4′aH-3,7′:2′,7′′-dispiroter[quinoline] (PIN)		1′H,2′′H,3H,4′aH-7,2′:7′,3′′-dispiroter[quinoline]

7-oxa-2,3′:7′,7′′-dispiroter[bicyclo[4.1.0]heptane] (PIN)

6,6′,6′′,8,8′,8′′-hexaoxa-2,7′:2′,7′′-dispiroter[bicyclo[3.2.1]octane] (PIN)

P-24.5.1 Monospiro ring systems with different ring components, at least one being a polycyclic ring system to which skeletal replacement (‘a’) nomenclature does not apply, are formed by placing the ring component names in alphanumerical order within square brackets. The position of the spiro atom is denoted by appropriate locants separated by a comma and placed between the names of the two ring components. Locants of the second ring component are primed and thus any locants needed to name it are placed in square brackets. Indicated hydrogen (see P-14.7) is cited in front of the name if needed in the complete structure (P-24.3.2).

Note: In naming polycyclic spiro ring systems consisting of different ring components, the first ring to be cited is determined by alphabetical order and not by seniority of the rings or ring systems.

Examples:

spiro[cyclohexane-1,1′-indene] (PIN)
(no indicated hydrogen is required, see P-24.3.2)

spiro[piperidine-4,9′-xanthene] (PIN)
(no indicated hydrogen is required, see P-24.3.2)

1′H-spiro[imidazolidine-4,2′-quinoxaline] (PIN)

Note: This treatment of skeletal replacement (‘a’) prefixes was introduced in the 1999 publication on the nomenclature of spiro compounds (ref. 8).

Examples:

	not
(I)		(II)
correct		incorrect
3-thiaspiro[bicyclo[2.2.2]octane-2,9′-fluorene] (I) (PIN) [not 2-thiaspiro[bicyclo[2.2.2]octane-3,9′-fluorene] (II)]

Note: The format of the ‘correct’ name given in SP-4.1 in ref. 8, spiro[fluorene-9,2′-[3]thiabicyclo[2.2.2]octane], is no longer recommended.

	not
(I)		(II)
correct		incorrect
2′,12′-dioxaspiro[bicyclo[2.2.1]heptane-2,1′-cyclododecane] (I) (PIN) [not 1′,3′-dioxaspiro[bicyclo[2.2.1]heptane-2,2′-cyclododecane] (II)]

Note: The format of the ‘correct’ name given in SP-4.1, ref. 8, spiro[bicyclo[2.2.1]heptane-2,1′-[2,12]dioxacyclododecane], is no longer recommended.

P-24.5.3 Alphanumerical order, as described in Section P-14.5 and P-14.6, is used when necessary. When Roman letters are inadequate to distinguish alphabetically between two ring components, criteria based on italic fusion letters and numbers, heteroatom locants, and von Baeyer descriptor numbers are used, as appropriate. All locants present in bicyclic fused benzo ring component or Hantzsch-Widman named component are placed in brackets (without primes for the second component, see third and fourth examples below).

Examples:

1H,2′H-spiro[benzo[g]isoquinoline-8,9′-benzo[h]isoquinoline] (PIN)
(benzo[g].... before benzo[h]....)

2′H,5H-spiro[thieno[2,3-b]furan-4,3′-thieno[3,2-b]furan] (PIN)
( ...[2,3-b]... before ...[3,2-b]...)

spiro[[1,2]benzodithiole-3,2′-[1,3]benzodithiole] (PIN)
(1,2-benzo... before 1,3-benzo...)

spiro[[3,1]benzoxazine-7,6′-[2,3]benzoxazine] (PIN)
(3,1-benzoxazine before 2,3-benzoxazine)

P-24.5.4 In the case of ring systems modified by skeletal replacement (‘a’) nomenclature, P-24.5.1 and P-24.5.3 are applied to name the ring system before skeletal replacement (‘a’) nomenclature is applied as described in P-24.5.2. The names of ring components modified by skeletal replacement (‘a’) nomenclature must not be used; thus P-24.5.3 must be applied when required.

Examples:

spiro[bicyclo[2.2.2]octane-2,3′-bicyclo[3.2.1]octane] (PIN)
[bicyclo[2.2.2]octane is cited before bicyclo[3.2.1]octane]

	not
(I)		(II)
correct		incorrect
3,3′-dioxaspiro[bicyclo[2.2.2]octane-2,6′-bicyclo[3.2.1]octane] (I) (PIN) [not 2,3′-dioxaspiro[bicyclo[2.2.2]octane-3,6′-bicyclo[3.2.1]octane] (II)]

P-24.6 UNBRANCHED POLYSPIRO RING SYSTEMS WITH DIFFERENT RING COMPONENTS ONE BEING A POLYCYCLIC RING SYSTEM

Unbranched polyspiro ring systems with at least two different ring components and at least one of which is a polycyclic ring are named by placing the ring component names in order of their occurrence in the structure beginning with the terminal ring component lower in alphabetical order and enclosing within square brackets. A nondetachable prefix indicating the number of spiro atoms (‘dispiro’, ‘trispiro’, etc.) is placed in front of the enclosed ring component names. Locants of the first cited ring component are unprimed, the next ring component is primed, and so on. In a complete name, all locants present in a bicyclic fused benzo ring component or Hantzsch-Widman named component are placed in brackets (without primes for the second and subsequent components, see fourth example below). The positions of the spiro atoms are indicated by the appropriate pair of locants separated by a comma and placed between each pair of component ring system names. Indicated hydrogen is used as needed and cited in front of the ‘dispiro’, ‘trispiro’, etc., prefix. If both terminal ring systems are the same, the order for citation of ring system components is determined by comparing the pair of second ring components from the end of the structure, and so on (See section SP-5 in ref. 8 for further discussion).

Examples:

dispiro[fluorene-9,1′-cyclohexane-4′,1′′-indene] (PIN)

2′′H,4′′H-trispiro[cyclohexane-1,1′-cyclopentane-3′,3′′-cyclopenta[b]pyran-6′′,1′′′-cyclohexane] (PIN)

7′-azadispiro[fluorene-9,2′-bicyclo[2.2.1]heptane-5′,1′′-indene] (PIN)

3,6-dioxadispiro[bicyclo[2.2.1]heptane-2,2′-[1,4]dioxane-5′,2′′-pyran] (PIN)

Example:

	not		not
(I)		(II)		(III)
correct		incorrect		incorrect
dispiro[[1,3]dioxolane-2,3′-bicyclo[3.2.1]octane-6′,2′′-[1,3]dioxolane] (I) (PIN) [not dispiro[[1,3]dioxolane-2,6′-bicyclo[3.2.1]octane-3′,2′′-[1,3]dioxolane] (II); the locant sets 2,2′′,3′,6′ are identical, but in the name the order of citation 2,3′,6′,2′′ in the PIN (I) is lower than 2,6′,3′,2′′ in (II)] [not dispiro[bicyclo[3.2.1]octane-3,2′:6,2′′-bis([1,3]dioxolane) (III) (see P-24.4.2)]

P-24.7 BRANCHED POLYSPIRO RING SYSTEMS

When three or more components are spirofused to another single component, the system is described as a branched spirofused system. Terminal components have only one spiro atom.

P-24.7.1 When a central ring component is spirofused to three or more identical terminal ring components, the central ring component is cited first and its locants are unprimed. The terminal ring components are cited with the appropriate multiplicative prefix (‘tris’, ‘tetrakis’, etc.) and their locants are primed, double primed, etc., in accordance with the lowest possible locants of the spiro atoms of the central ring component. The spiro atoms are indicated by pairs of locants separated by a colon. Indicated hydrogen is cited as necessary in front of the appropriate spiro prefix.

Example:

trispiro[[1,3,5]trithiane-2,2′:4,2′′:6,2′′′-tris(bicyclo[2.2.1]heptane)] (PIN)

Examples:

(1)
	trispiro[[1,3]benzodioxole-2,1′-cyclohexane-2′,2′′:4′,2′′′-bis([1,3]dioxolane)] (PIN)

(2)
	trispiro[cyclohexane-1,2′-[1,5]dithiocane-6′,1′′-cyclopentane-4′,2′′′-indene] (PIN) (the cyclohexane ring must be spirofused at position ‘2’ of the dithiocane central ring component)

(3)		not
	(I)		(II)
	correct		incorrect
	3H-trispiro[[1,4]benzodioxine-2,1′-cycloheptane-3′,1′′:5′,1′′′-bis(cyclopentane)] (I) (PIN) {not 3H-trispiro[[1,4]benzodioxine-2,1′-cycloheptane-4′,1′′:6′,1′′′-bis(cyclopentane)] (II)}

(4)		not
	(I)		(II)
	correct		incorrect
	trispiro[cyclopentane-1,1′-cyclohexane-3′,2′′-imidazole-5′,1′′′-indene] (I) (PIN) [not trispiro[cyclopentane-1,1′-cyclohexane-5′,2′′-imidazole-3′,1′′′-indene] (II)

P-24.7.3 If no decision can be attained by application of P-24.7.2 and if there is a choice of locants, the lowest set of locants for all spiro atoms is selected when compared as a set in increasing numerical order, and, if still undecided, in their order of their citation in the name. If a choice still remains, criteria about heteroatoms and indicated hydrogen are taken into consideration (see Section SP-3.2 in ref. 8).

Example:

		not
	(I)		(II)
	correct		incorrect
nor		nor
	(III)		(IV)
	incorrect		incorrect
	trispiro[bis(cyclohexane)-1,2′:6′,1′′-[1,5]dithiocane-4′,2′′′-indene] (I) (PIN) [not trispiro[bis(cyclohexane)-1,8′:4′,1′′-[1,5]dithiocane-6′,2′′′-indene] (II) nor trispiro[bis(cyclohexane)-1,4′:8′,1′′-[1,5]dithiocane-2′,2′′′-indene] (III); nor trispiro[bis(cyclohexane)-1,6′:2′,1′′-[1,5]dithiocane-8′,2′′′-indene] (IV)]

P-24.7.4 If additional components are spiro-fused to a branched polyspiro compound as described in P-24.7.1 to P-24.7.3, the following criteria are applied in order:

(a) Any monocyclic ring components spirofused together including skeletal replacement (‘a’) prefixes, if any, are named (P-24.2) as a unit containing the maximum number of monocyclic ring components. The unit is used as a component for further spiro-fusion (see P-24.6). To help indicate the composite nature of the name braces are used (instead of brackets) after the initial polyspiro prefix to enclose components at least one of which is already spiro-fused;

(b) If there is not a polyspiro system of monocyclic ring components or the system cannot be named by the normal spiro-fusion described previously the largest spiro system is named and treated as a unit for further spiro-fusion. The priming of the unit is continued to the rest of the name.

Examples:

trispiro{bis(cyclohexane)-1,4′:1′′,6′-furo[3,4-d][1,3]oxathiole-2′,14′′′-[7]oxadispiro[5.1.5⁸.2⁶]pentadecane} (PIN)
pentaspiro[tetracyclohexane-1,2′(5′H):1′′′,5′:1′′′′,4′′(6′′H):1′′′′′,6′′-furan-3′(4′H),2′′-furo[3,4-d][1,3]oxathiole]
(the CAS index name; note that multiple primes are not divided into groups of three)

1λ⁵,3λ⁵,5λ⁵,7λ⁵-tetraspiro{tetraspiro[2,4,6,8,9,10-hexathia-1,3,5,7- tetraphosphaadamantane-1,2′:3,2′′:5,2′′′:7,2′′′ ′-
tetrakis([1,3,2]oxathiaphosphetane)-4′,7′′′ ′′:4′′,7′′′ ′′′:4′′′,7′′′ ′′′ ′:4′′′ ′,7′′′ ′′′ ′′-tetrakis(pyrano[2,3-c]acridine)} [PIN, see SP-6.4 (b) in ref 8]
octaspiro[2,4,6,8,9,10-hexathia-1,3,5,7-tetraphosphatricyclo[3.3.1.1^3,7]decane- 1,2′λ⁵:3,2′′λ⁵:5,2′′′λ⁵:7,2′′′′λ⁵-
tetrakis[1,3,2]oxathiaphosphetane- 4′,7′′′′′:4′′,7′′′′′′:4′′′,7′′′′′′′:4′′′′,7′′′′′′′′-tetrakis[7H]pyrano[2,3-c]acridine]
(the CAS index name; note that multiple primes are not divided into groups of three)

trispiro{1-oxaspiro[2.3]hexane-2,3′:4,3′′:5,3′′′-tris(tetracyclo[3.2.0.0^2,7.0^4,6]heptane)} (PIN)

The λ-convention, characterized by the symbol λⁿ, is used to identify heteroatoms with nonstandard bonding numbers (see P-14.1). The symbol is placed at the front of the complete name or before the skeletal replacement (‘a’) prefix for the atom to which it refers.

P-24.8.1 Spiro ring systems with only monocyclic ring components
P-24.8.2 Monospiro ring systems with two identical polycyclic ring components
P-24.8.3 Spiro ring systems with three identical components and one nonstandard spiro atom
P-24.8.4 Monospiro ring systems with different ring components, at least one of which is a polycyclic ring with a nonstandard spiro atom
P-24.8.5 Unbranched polyspiro ring systems with different ring components, at least one of which is a polycyclic ring with at least one nonstandard spiro atom
P-24.8.6 Branched spiro ring systems with at least one polycyclic ring component

P-24.8.1.1 Heteroatoms having nonstandard bonding numbers receive lowest locants in accordance with the numbering of the corresponding spiro ring system.

Examples:

7λ⁵-phosphaspiro[3.5]nonane (PIN)

7λ⁵-phospha-2-silaspiro[3.5]nonane (PIN)

4λ⁴-thiaspiro[3.5]nonane (PIN)

Example:

2λ⁶,4λ⁴-dithiaspiro[5.5]undecane (PIN)

P-24.8.1.3.1 Ring systems consisting of three monocyclic rings and one nonstandard spiro atom (e.g., a λ⁶ spiro atom) are named by placing the prefix ‘spiro’ before the name corresponding to an alicyclic system with the same total number of skeletal atoms in the spiro ring system. Heteroatoms are indicated by ‘a’ prefixes and the nonstandard bonding number by the ‘λ’ symbol (see P-14.1). In the von Baeyer spiro descriptor, the locant of the spiro atom is used as a superscript number to indicate each time the spiro atom is revisited.

Example:

1,4,6,9,10,13-hexaoxa-5λ⁶-thiaspiro[4.4⁵.4⁵]tridecane (PIN)

Example:

3λ⁶-thiaspiro[2.4³.5³]dodecane (PIN)

(a) Low numbers are selected for spiro atoms;

Example:

	not		not
(I)		(II)		(III)
correct		incorrect		incorrect
7λ6-thiatrispiro[2.0.2.27.37.24.33]heptadecane (I) (PIN) [not 3λ6-thiatrispiro[2.2.0.27.36.23.33]heptadecane (II); nor 3λ6-thiatrispiro[2.3.2.0.210.39.23]heptadecane (III)]

(b) Low numbers are selected for spiro atoms connecting three rings;

Example:

	not		not
(I)		(II)		(III)
correct		incorrect		incorrect
3λ6-thiatrispiro[2.2.26.2.211.23.23]heptadecane (I) (PIN) [not 6λ6-thiatrispiro[2.2.26.26.2.213.23]heptadecane (II); nor 11λ6-thiatrispiro[2.2.26.2.211.211.23]heptadecane (III)]

(c) Low numbers are selected for the von Baeyer spiro descriptor in order of citation;

Example:

	not
(I)		(II)
correct		incorrect
5λ6-thiatrispiro[2.1.1.27.15.25.23]tetradecane (I) (PIN) [not 5λ6-thiatrispiro[2.1.1.27.25.25.13]tetradecane (II)]

P-24.8.2 Monospiro ring systems with two identical polycyclic ring components

The λⁿ symbol is placed at the front of the complete name formed from the names of two identical ring system components including the heteroatoms in their name; the λⁿ symbol is preceded by the lowest locant denoting the spiro atom. If indicated hydrogen atoms are required, they are placed before the λ symbol. When there is a choice, the locant of the first cited component is used for indicated hydrogen.

Note: The ‘lowest locant’ is used as the criterion for identification of the λⁿ spiro atom rather than ‘least primed locant‘ as used in SP-7 in ref. 8.

Examples:

2λ⁴,2′-spirobi[[1,3,2]benzodioxathiole] (PIN)

1H-2λ⁵,2′-spirobi[[1,3,2]benzodiazaphosphinine] (PIN)

P-24.8.3 Spiro ring systems with three identical components and one nonstandard spiro atom.

Ring systems composed of three identical polycyclic ring components and only one spiro atom are named by placing the prefix ‘spiroter’ before the name of the polycyclic component enclosed in square brackets. The three spiro locants are cited at the front of the name preceded by the λ symbol with its locant that must be the lowest of the three denoting the spiro atom.

Example:

2λ⁶,2′,2′′-spiroter[[1,3,2]benzodioxathiole] (PIN)

P-24.8.4 Monospiro ring systems with different ring components at least one of which is a polycyclic ring with a nonstandard spiro atom

P-24.8.4.1 Monospiro ring systems composed of two different ring systems and a spiro heteroatom with a nonstandard bonding number are named by placing the prefix spiro in front of the names of the components cited in alphabetical order and with appropriate spiro locants. The lowest locant (unprimed) is used to denote the spiro fusion and the λ symbol is placed at the front of the name. Indicated hydrogen, if necessary, is added in front of the λ symbol. Any additional atom with a nonstandard bonding number is treated as a part of the name of the heterocycle; the λ symbol is cited with the lowest locant denoting the spiro atom.

Examples:

3H-2λ⁵-spiro[[1,3,2]benzoxazaphosphole-2,2′-[1,3,5,2]triazaphosphinine] (PIN)

3H-1′λ⁵-spiro[[1,4,2]oxazaphosphole-2,1′-[2,8,9]trioxa[1]phosphaadamantane] (PIN)
3H-1′λ⁵-spiro[[1,4,2]oxazaphosphole-2,1′-[2,8,9]trioxa[1]phosphatricyclo[3.3.1.1^3,7]decane]

Note 2: The double set of brackets in this name occurs because the spiro name requires them and brackets are used to enclose locants belonging to ring component names (see P-16.5.2.2). The use of brackets in the name of the first cited component is a change from SP-4 of ref. 8.

3H-2λ⁵,5′λ⁵-spiro[[1,3,2]benzoxazaphosphole-2,2′-[1,3,2,5]diazadiphosphinine] (PIN)

P-24.8.4.2 When three ring components represented by two different individual rings, are present, the name of the ring component that is cited second is placed in parentheses to highlight this unusual situation. The λⁿ symbol is placed at the front of the name, preceded by the lowest locant denoting the spiro atom.

Example:

2λ⁶-spiro[[1,3,2]benzodioxathiole-2,2′-([1,2,3]benzoxadithiole)-2,5′′-dibenzo[b,d]thiophene] (PIN)

P-24.8.4.3 Two identical ring components are denoted by the prefix ‘bis’. The λⁿ symbol is placed at the front of the name, preceded by the lowest locant denoting the spiro atom.

Example:

2λ⁶-spiro[bis([1,3,2]benzodioxathiole)-2,2′′:2′,2′′-[1,2,3]benzoxadithiole] (PIN)

P-24.8.5 Unbranched polyspiro ring systems with different ring components at least one of which is a polycyclic ring with at least one nonstandard spiro atom

Unbranched polyspiro ring systems with different ring components at least one of which is a polycyclic ring system with at least one nonstandard spiro atom are named using the method described in P-24.6. The λ-symbol denoting a spiro junction is associated with the lowest locant and placed in front of the name; it is preceded by indicated hydrogen(s), as needed.

Example:

1′H,3′H-1λ⁴,1′′λ⁴-dispiro[thiane-1,2′-benzo[1,2-c:4,5-c′]dithiophene-6′,1′′-thiolane] (PIN)

If two or more different terminal ring components are spirofused to a central ring component, the alphabetically earliest is cited first with multiplicative prefixes, if appropriate, followed by the central ring component and the remaining terminal ring components in alphabetical order. The λ symbol is placed at the front of the complete name and is denoted by the lowest spiro locant; it is preceded by indicated hydrogen, if needed.

Example:

1′′λ⁶-dispiro[bis([1,3,2]benzodioxathiole)-2,1′′:2′,1′′-thiopyran-4′′,1′′′-cyclopentane] (PIN)

When there is a choice for locants, the lowest set of locants for all spiro atoms is selected, first by comparing them as a set in increasing numerical order, and, if still undecided, in the order of citation in the name. If a choice still remains, criteria involving the heteroatoms and indicated hydrogen atoms are taken into consideration (see Section SP-3.2 and SP-1.8 in ref. 8).

Example:

	not		not
(I)		(II)		(III)
correct		incorrect		incorrect
1′λ4-trispiro[cyclopentane-1,5′-[1,4]dithiane-2′,2′′-indene-1′,1′′-thiophene] (I) (PIN) [not 4′λ4-trispiro[cyclopentane-1,2′-[1,4]dithiane-5′,2′′-indene-4′,1′′′-thiophene] (II)] nor 1′λ4-trispiro[cyclopentane-1,3′-[1,4]dithiane-6′,2′′-indene-1′,1′′′-thiophene] (III)]

P-25 FUSED AND BRIDGED FUSED RING SYSTEMS

P-25.0 Introduction
P-25.1 Names of hydrocarbon parent ring components
P-25.2 Names of heterocyclic parent ring components
P-25.3 Constructing fusion names
P-25.4 Bridged fused ring systems
P-25.5 Limitations of fusion nomenclature: three components ortho- and peri-fused together
P-25.6 Fused ring systems with skeletal atoms with nonstandard bonding numbers
P-25.7 Double bonds, indicated hydrogen, and the δ-convention
P-25.8 Parent components in decreasing order of seniority (partial lists)

This section is based on the document entitled ‘Nomenclature of Fused and Bridged Fused Ring Systems, IUPAC Recommendations 1998’ (ref. 4).

In nomenclature, fusion is the operation that creates a common bond between two rings, each ring contributing one bond and the two atoms directly attached to the bond. This type of fusion is called ortho- or ortho- and peri-fusion if two adjacent bonds are involved. Atoms common to two or more rings are termed ‘fudion’ atoms. The term fusion is also used to describe the operation creating a common atom between two rings or ring systems, each contributing one atom. This type of fusion is called spirofusion (see P-24.1). Traditionally, ortho- and ortho- and peri-fusion were simply called fusion and the resulting polycyclic systems were referred to as fused ring systems or fused ring compounds. The term ‘spirofusion’ is new in nomenclature, and to avoid ambiguity ‘fusion’ should not be used without the prefix ‘spiro’ when ‘spirofusion’ is intended.

	+
benzene (PIN)		benzene (PIN)		naphthalene (PIN)

	+
naphthalene (PIN)		benzene (PIN)		1H-phenalene (PIN)

	+
1H-indene (PIN)		1H-indene (PIN)		1,1′-spirobi[indene]	(PIN)

This section deals with fused (ortho- and ortho- and peri-fused) ring systems and bridged fused (ortho- and ortho- and peri-fused) ring systems. Spirofusion is described in Section P-24. This section is intended only as an introduction to the vast field of fusion nomenclature discussed in the document entitled ‘Nomenclature of Fused and Bridged Fused Ring Systems’ (ref. 4). The principles presented herein use rather simple examples; for more complex ring systems the publication noted above or the ‘Ring Systems Handbook’, published by the Chemical Abstracts Service (ref. 22), should be consulted. Changes from previous rules are highlighted.

P-25.1 NAMES OF HYDROCARBON PARENT RING COMPONENTS

P-25.1.1 Retained names for hydrocarbons used for parent ring components and as attached ring components
P-25.1.2 Systematically named hydrocarbon parent components

Retained names (also called trivial names) for polycyclic hydrocarbons are listed in Table 2.7, in decreasing order of seniority for being chosen as parent components in fusion nomenclature. Their numbering is indicated as the result of the application of the specific criteria used to number fused ring systems described in Section P-25.3.3.

Names for some hydrocarbon parent components having the maximum number of noncumulative double bonds (called mancude ring systems) and having at least two rings of five or more ring members are systematically formed using a prefix and an ending or term representing the nature and arrangement of the component rings. Rules for numbering are described in Section P-25.3.3.

P-25.1.2.1 Polyacenes
P-25.1.2.2 Polyaphenes
P-25.1.2.3 Polyalenes
P-25.1.2.4 Polyphenylenes
P-25.1.2.5 Polynaphthylenes
P-25.1.2.6 Polyhelicenes
P-25.1.2.7 Ace...ylenes

Examples:

tetracene (PIN)
(formerly naphthacene)

pentacene (PIN)

pentaphene (n = 5) (PIN)

hexaphene (n = 6) (PIN)

Examples:

pentalene (PIN)

octalene (PIN)

Examples:

biphenylene (PIN)
(not diphenylene)

triphenylene (PIN)

Examples:

dibenzo[b,h]biphenylene (PIN)
(not dinaphthylene)

trinaphthylene (PIN)

Note: The definition, orientation, and numbering of polyhelicenes was changed in the comprehensive fused ring nomenclature document ‘Nomenclature of Fused and Bridged Fused Ring Systems’ (ref. 4). The series begins with six rings and not five rings as indicated in the 1993 Guide (R-2.4.1.3.6 in ref. 2) and in the Glossary of Class Names (ref. 23). The new orientation and numbering are presented in Section P-25.3.3.1.1.

Example:

hexahelicene (PIN) (new orientation and numbering)		Note: Orientation and numbering no longer recommended; but still in use by CAS for the ring system having the name phenanthro[3,4-c]phenanthrene, ref. 22)

Examples:

acenaphthylene (PIN)

aceanthrylene (PIN)

acephenanthrylene (PIN)

P-25.2.1 Retained names used for parent components and as attached components
P-25.2.2 Names formed systematically using endings and prefixes used for parent components and attached components

Ring systems are arranged in decreasing order of seniority for parent compounds in accordance with the seniority order described in Section P-25.3.2.4 and exemplified in Section P-25.8.1.

Skeletal replacement nomenclature, as described in Section P-15.5.2 , is used to replace O by S, Se, and Te of chromene, isochromene, and xanthene (PIN) to generate names for chalcogen analogues of these ring systems (see Table 2.8). Some names listed in Table 2.8 can be modified by a system of replacement specific to some nitrogen-containing compounds, in which N is replaced by As or P. The modified names are listed in Table 2.9; the modifiable compounds are marked by the symbol * in Table 2.8. Rules for numbering are described in Section P-25.3.3.

P-25.2.2.1 Heteromonocyclic parent components
P-25.2.2.2 Heteranthrene components
P-25.2.2.3 Pheno...ine components
P-25.2.2.4 Heteromonocyclic components fused to a benzene ring

P-25.2.2.1.1 Heteromonocyclic rings with three through ten ring members having the maximum number of noncumulative double bonds (called mancude ring systems) are used as parent components as well as attached components. Retained names are given in Table 2.2. Hantzsch-Widman names are discussed in P-22.2.2.

P-25.2.2.1.2 Names of heteromonocyclic parent components with more than ten ring members used in fusion nomenclature are discussed in this subsection; they are used only in fusion nomenclature (see also P-22.2.4). The preferred IUPAC names for such rings are ‘polyene’ names (see P-22.2.4).

A heteromonocyclic parent component having more than ten members and the maximum number of noncumulative double bonds (a mancude ring system) may be named by changing the ending ‘ane’ of the corresponding saturated heteromonocycle (see P-22.2.4) to ‘ine’. Their locants are cited in front of the name following indicated hydrogen, if any, in the order of the appearance of the corresponding replacement (‘a’) prefixes in the name.

For examples of fusion compounds including this type of heteromonocyclic parent component, see P-25.2.2.4.

Examples:

1,8-dioxacyclooctadecine
1,8-dioxacyclooctadeca-2,4,6,9,11,13,15,17-octaene (PIN)

2H-1,4,8,11-oxatriazacyclotetradecine
1-oxa-4,8,11-triazacyclotetradeca-3,5,7,9,11,13-hexaene (PIN)

A heterotricyclic parent component consisting of two benzene rings fused to a 1,4-dihetera-benzene in which the heteroatoms are the same is named by attaching the appropriate ‘a’ prefix to the ending ‘anthrene’ (derived from anthracene), with elision of a letter ‘a’. The allowed heteroatoms are O, S, Se, Te, P, As, Si, and B. When the heteroatoms are nitrogen atoms, the component is named ‘phenazine’ (a retained name). The numbering is standard, as shown. Rules for numbering are described in Section P-25.3.3.

Examples:

X = O oxanthrene (PIN)

X = S thianthrene (PIN)

X = Se selenanthrene (PIN)

X = Te telluranthrene (PIN)

X = N phenazine (PIN)
(a retained name)

X = P phosphanthrene (PIN)

X = As arsanthrene (PIN)

X = B boranthrene (PIN)

X = SiH silanthrene (PIN)

A heterotricyclic parent component consisting of two benzene rings fused to a 1,4-dihetera-benzene in which the heteroatoms are different is named by adding the prefix ‘pheno’ (derived from phenanthrene) to the appropriate Hantzsch-Widman name (see P- 22.2.2), eliding the ‘o’ before a following vowel. Numbering is standard and depends on the nature of the heteroatoms. Rules for numbering are described in Section P-25.3.3.

Examples:

X = O phenoxazine (10H-isomer shown; the PIN is 10H-phenoxazine)

X = S phenothiazine (10H-isomer shown; the PIN is 10H-phenothiazine)

X = Se phenoselenazine (10H-isomer shown; the PIN is 10H-phenoselenazine)

X = Te phenotellurazine (10H-isomer shown; the PIN is 10H-phenotellurazine)

X = P phenazaphosphinine (PIN)
phenophosphazine (see refs. 2, 4)

X = As phenazarsinine (PIN)
phenarsazine (see refs. 2, 4)

X = S phenoxathiine (PIN)

X = Se phenoxaselenine (PIN)

X = Te phenoxatellurine (PIN)

X = PH phenoxaphosphinine (PIN, 10H-isomer shown)
phenoxaphosphine (10H-isomer shown)

X = AsH phenoxarsinine (PIN, 10H-isomer shown)
phenoxarsine (10H-isomer shown)

X = SbH phenoxastibinine (PIN, 10H-isomer shown)
phenoxantimonine (10H-isomer shown)

X = AsH; and S instead of O phenothiarsinine (PIN, 10H-isomer shown)
phenothiarsine (10H-isomer shown)

If the initial identified preferred components for naming a fused ring system include an isolated benzo component (i.e. not forming part of a component with a retained name such as quinoline or anthracene) ortho-fused to a heteromonocyclic component of five or more members, these two components are treated together as a one-component unit (a benzoheterocycle). See P-25.3.5 for the use of benzoheterocycles in fusion nomenclature and limitations on its use. It is named by placing the locant(s) indicating the position(s) of the heteroatom(s) at the front of the name consisting of the fusion prefix ‘benzo’ followed by a retained name, a Hantzsch-Widman systematic name, or a name formed by skeletal replacement (‘a’) nomenclature as described in P-25.2.2.1.2. The locants cited correspond to the full bicyclic structure. As in Hantzsch-Widman names, locants are placed in the order corresponding to the order of citation of the heteroatoms in the heterocyclic component. The locant ‘1’ is always assigned to the atom of the heterocyclic component next to a fusion atom. Heteroatoms are allocated lowest locants as a set, without regard to kind; if there is a choice, lowest locants are assigned in accordance with the seniority of the ‘a’ prefixes (see Table 2.4). In general nomenclature locants may be omitted when the name is unambiguous; for preferred IUPAC names locants must be cited. The letter ‘o’ of the ‘benzo’ prefix is elided when followed by a vowel. Indicated hydrogen is placed at the front of the name, when required.

‘Benzo names’ offer several advantages. They are simpler in the sense that they do not require fusion descriptors. However, their primary advantage is in their use as components of fusion names; they provide a larger portion of structure and remove one full level of locants in the construction of names for larger heterocyclic fused ring systems.

Examples:

3-benzoxepine (PIN)
benzo[d]oxepine

4H-3,1-benzoxazine (PIN)
4H-benzo[d][1,3]oxazine

1-benzofuran (PIN)
benzofuran

2-benzofuran (PIN)
isobenzofuran
benzo[c]furan

5,12-benzodioxacyclooctadecine (PIN)
benzo[m][1,8]dioxacyclooctadecine

1H-3-benzazacycloundecine (PIN)
1H-benzo[h][1]azacycloundecine
1H-3-benzoazacycloundecine

9,2,5-benzoxathiaazacyclododecine (PIN)
(not 2,9,6-benzoxathiaazacyclododecine; the locant set 2,5,9 is lower than 2,6,9)
benzo[j][1,8,5]oxathiaazacyclododecine

P-25.3.1 Definitions, terminology, and general principles
P-25.3.2 Constructing two-component fusion names
P-25.3.3 Numbering of fused ring systems
P-25.3.4 Constructing polycomponent fusion names
P-25.3.5 Heteromonocyclics fused to a benzene ring
P-25.3.6 Identical attached components
P-25.3.7 Multiparent ring systems
P-25.3.8 Omission of locants in fusion descriptors

P-25.3.1.1 Definitions

P-25.3.1.1.1 ortho-Fused. Two rings that have only two atoms and one bond in common are said to be ortho-fused

Example:

The two benzene rings of the naphthalene skeletal ring system are ortho-fused.

Example:

The phenylene skeleton is composed of three benzene rings, each ortho- and peri-fused to the other two

P-25.3.1.1.4 Peripheral atom. Any atom that forms part of the outer perimeter of a fused ring system.

P-25.3.1.1.5 Interior atom. Any fusion atom that is not peripheral.

P-25.3.1.2 Terminology

P-25.3.1.2.1 Components of a fused ring system. Fusion components are mancude or ring systems that can be named without the application of any fusion nomenclature principles. Fused ring systems that do not have such a name are named by joining together the names of appropriately selected fusion components.

P-25.3.1.2.2 Parent component. The parent component according to the terminology of the 1998 recommendations (ref. 4) (referred to as base component in the 1979 publication, ref. 1; and principal component in the 1993 Recommendations, ref. 2) is the one with highest seniority according to the criteria given in P-25.3.2.4. A parent component may be mono- or polycyclic, but it must be a mancude ring or ring system. Its name is never modified and is cited last in the name of the fused system.

P-25.3.1.2.3 Attached component. The components of a fused ring system not included in the parent component are called attached components. The attached components are called first-order, second-order, etc. attached components when they correspond to the first, second, etc. attached component reached when moving away from the parent component across fusion sites. An attached component may be mono- or polycyclic, but it must be a mancude ring or ring system. Fusion sites are bold lines in the following examples.

Example:

P-25.3.1.2.4 Interparent components. In a system that consists of two (or more) parent components ortho- or ortho- and peri-fused to the same attached component, the latter is called an interparent component. Likewise, if two (or more) parent components are fused to three or more appropriately attached components, there will be two first- order interparent components and a second-order interparent component. Fourth-, fifth-, etc. order components may be present in more complex systems.

Example:

P-25.3.1.3 General principles

‘ortho-Fused’ or ‘ortho- and peri-fused’ polycyclic ring systems with the maximum number of noncumulative double bonds (mancude) that have no accepted retained or systematic name described in sections P-25.1 and P-25.2 are named by prefixing to the name of a component ring or ring system (the parent component) designations of the other component(s) (attached components). For fused ring systems in preferred IUPAC names, see P-52.2.4.

The parent component is selected by applying criteria of seniority as described in P- 25.3.2.4 below. In a fusion name, the name of the parent component is that of the component itself. The names of attached components are formed by replacing the last letter ‘e’ by ‘o’ in the name of the component, i.e., indeno from indene (or by adding the letter ‘o’ when no final letter ‘e’ is present, i.e., pyrano from pyran) or by other means described in P-25.3.2.2 below. There is no elision of the final letter ‘o’ or ‘a’ before a vowel (see Rule FR-4.7, ref. 4).

Locants that describe structural features of components, such as positions of heteroatoms, are kept with the name of the component and are enclosed within square brackets.

Note: In preferred IUPAC names the elision of the final letter ‘o’ of ‘acenaphtho’, ‘benzo’, ‘naphtho’ and ‘perylo’ and the final letter ‘a’ of the monocyclic prefixes ‘cyclopropa’, ‘cyclobuta’, etc. is not recommended as indicated in FR-4.7 of the 1998 publication, ref. 4; hence, ‘benzo[g]isoquinoline’ rather than ‘benz[g]isoquinoline’. However this elision as recommended in the 1979 recommendations (Rule A-21.4, ref. 1) may be continued in general nomenclature.

Isomers are distinguished by lettering, continuously, each peripheral side of the parent component (including sides whose locants are distinguished by letters, for example, 2a,3a) using the italic letters a, b, c, etc., beginning with a for the side numbered ‘1,2’, b for ‘2,3’ etc.. To the letter as early in the alphabet as possible that denotes the side where the fusion occurs are prefixed, if necessary, the numbers of the positions of attachment of the other component. These numbers are chosen to be as low as is consistent with the numbering of the compound and their order conforms to the direction of lettering of the parent component. In this document these letters and numbers are placed within the structure of the ring or ring system.

Examples:

azulene (PIN) + naphthalene (PIN) (parent component) (attached component)	naphtho[1,2-a]azulene (PIN)

azulene (PIN) + naphthalene (PIN) (parent component) (attached component)	naphtho[2,1-a]azulene (PIN)

The numbers and letters, separated by commas when required, are enclosed in square brackets and placed immediately after the designation of the attached component; there is no space or hyphen either preceding or following the brackets. Hyphens separate the two parts of a fusion descriptor, i.e., numbers and italicized letters. This expression merely defines the manner of fusion of the components. Indicated hydrogen atoms are added to the names, as required, using locants characterizing the fused system.

Examples:

selenopheno[2,3-b]selenophene (PIN)

selenopheno[3,4-b]selenophene (PIN)

selenopheno[3,2-b]selenophene (PIN)

Examples:

6H-pyrrolo[3,2,1-de]acridine

naphtho[2,1,8-mna]acridine

A component may be monocyclic or polycyclic. Systematic construction proceeds stepwise as follows.

P-25.3.2.1 Selecting and naming parent components for fusion nomenclature
P-25.3.2.2 Prefixes for attached components
P-25.3.2.3 Orientation of fused ring systems
P-25.3.2.4 Seniority criteria for selecting parent components
P-25.3.2.5 Assembling components and naming fused ring systems

P-25.3.2.1.1 Monocyclic hydrocarbons (annulenes).

Monocyclic parent components are named as [n]annulenes where n represent the number of carbon atoms. The series starts at n = 7, because the retained name ‘benzene’ is preferred for n = 6. The use of the name ‘annulene’ in fusion nomenclature was recommended in the 1993 Guide (see R-2.3.1.2, ref. 2) to obviate the potential ambiguity of using contracted traditional names, such as cycloheptene to denote 1,3,5-cycloheptatriene.

Examples:

1H-[7]annulene
(no longer cycloheptene as fusion component)
cyclohepta-1,3,5-triene (PIN)

[10]annulene
(no longer cyclodecene as fusion component)
cyclodeca-1,3,5,7,9-pentaene (PIN)

The retained names given in Table 2.2, except for ‘isothiazole’, ‘isoxazole’, ‘thiazole’, and ‘oxazole’, and Hantzsch-Widman names for unsaturated heteromonocycles (see P-22.2.2) are used as parent components in fusion nomenclature. The names ‘isothiazole’, ‘isoxazole’, ‘thiazole’, and ‘oxazole’, although permitted in general nomenclature, are not recommended for the names of components in preferred IUPAC fusion names. The Hantzsch-Widman names 1,2-thiazole, 1,2-oxazole, 1,3- thiazole, and 1,3-oxazole, respectively, must be used; the locants are enclosed in square brackets in the completed fusion name.

Heteromonocycles having more than ten members and the maximum number of noncumulative double bonds whose names are denoted by the ‘ine’ ending described in P-25.2.2.1.2 are used as parent components in preferred IUPAC fusion names.

Examples:

1,8-dioxacyclooctadecine
1,8-dioxacyclooctadeca-2,4,6,9,11,13,15,17-octaene (PIN)

2H-1,4,8,11-oxatriazacyclotetradecine
1-oxa-4,8,11-triazacyclotetradeca-3,5,7,9,11,13-hexaene (PIN)

P-25.3.2.2 Prefixes for attached components.

P-25.3.2.2.1 Monocyclic hydrocarbon prefixes for attached components other than ‘benzo’ are formed by dropping ‘ne’ from the name of the appropriate saturated monocyclic hydrocarbon. These names represent the form with the maximum number of noncumulative double bonds. There is no upper ring-size limit to this criterion.

Examples:

cyclopropa (preferred prefix)
(from cyclopropane, PIN)

cyclobuta (preferred prefix)
(from cyclobutane, PIN)

cyclopenta (preferred prefix)
(from cyclopentane, PIN)

cyclohepta (preferred prefix)
(from cycloheptane, PIN)
(not [7]annuleno)

cycloocta (preferred prefix)
(from cyclooctane, PIN)
(not [8]annuleno)

Examples:

pyrazolo (preferred prefix)
(from pyrazole, PIN)

selenopyrano (preferred prefix)
(from selenopyran, PIN)

thiepino (preferred prefix)
(from thiepine, PIN)

pentaleno (preferred prefix)
(from pentalene, PIN)

1,4,8,11-oxatriazacyclotetradecino (preferred prefix)
(from 1,4,8,11-oxatriazacyclotetradecine, see P-25.2.2.1.2, P-25.3.2.1.2)

Only the following contracted prefixes are retained for preferred IUPAC fusion names. The contracted prefixes acenaphtho, perylo, isoquino, and quino are retained but are only used in general nomenclature.

P-25.3.2.3.1 Drawing of ring structures

For the purpose of selecting parent components and for numbering of fused ring systems, the structures of fused ring compounds must be drawn in a specific manner according to a set of criteria that must be applied in order until a decision is reached. Individual rings of a polycyclic ‘ortho-fused’ or ‘ortho- and peri-fused’ hydrocarbon ring system are drawn in such a way so that as many as possible of the various individual rings are arranged in horizontal rows. If the compound requires distorted rings not shown below see P-25.3.2.3.2. Such rows are characterized by a horizontal axis that divides each individual ring into two approximate halves. Permitted shapes for three- to eight-membered rings are as follows:

a horizontal row and its horizontal axis

P-25.3.2.3.2 Distorted ring shapes

If a compound cannot be drawn only using the shapes shown in P-25.3.2.3.1 a distorted ring shape will be required. The distorted ring should be as small as possible.

	not
only preferred shapes used		distorted five-membered ring
	not
only preferred shapes used (the separation to show the seven-membered ring is not fused to the left hand ring does not count as a distortion)		distorted seven-membered ring
	not
distorted four-membered ring		distorted six-membered ring

P-25.3.2.3.3 Criteria for selection of the preferred orientation

Polycyclic fused ring systems are oriented in accordance with the following criteria considered in order until a decision is reached:

(a) maximum number of rings in a horizontal row;

Fused ring systems are drawn in order to achieve the maximum number of ortho-fused rings, with vertical common bonds, in a horizontal row. The relevant vertical bonds are always those furthest apart. If the correct orientation is not immediately apparent the horizontal row is bisected by a horizontal axis and a vertical axis to form four quadrants. Rings which are not bisected by the horizontal axis do not belong to the main row and are not considered in the counting of rings in the main row.

Examples:

3 rings in horizontal row

2 rings in horizontal row

(b) maximum number of rings in upper right quadrant;

In the preferred orientation, the maximum number of rings must appear above and to the right of the horizontal row (upper right quadrant). For this purpose, the center of the horizontal row is defined as the central common bond if there is an even number of rings in the row, and the center of the central ring if there is an odd number of rings. In counting rings in a quadrant those rings that are divided by an axis are considered as two halves, and a ring bisected by both axes, counts as four quarters (one in each quadrant). Rings that are bisected by the horizontal axis but are not directly ortho-fused to the main row are not considered when counting how many rings are in the horizontal row.

Examples:

	not
(correct orientation)		(incorrect orientation)
2 rings in horizontal row 2 rings in upper right quadrant		2 rings in horizontal row 1 ring in upper right quadrant
:
	not
(correct orientation)		(incorrect orientation)
2 rings in the horizontal row 3½ rings in upper right quadrant		2 (not 3) rings in the horizontal row 3 rings in upper right quadrant

Accordingly, phenanthrene (1½ rings in the upper right quadrant) is senior to phenalene [1 ring (two ½ rings) in the upper right quadrant].

(c) minimum number of rings in the lower left quadrant;

Example:

	not
(correct orientation)		(incorrect orientation)
3 rings in horizontal row 1¾ rings in upper right quadrant ¾ ring in lower left quadrant		3 rings in horizontal row 1¾ ring in upper right quadrant 1¾ ring in lower left quadrant

(d) maximum number of rings above the horizontal row.

Examples:

	not		not
(correct orientation)		(incorrect orientation)		(incorrect orientation)
3 rings in horizontal row 1¾ rings in upper right quadrant ¾ ring in lower left quadrant 3½ rings above horizontal row		3 rings in horizontal row 1¾ ring in upper right quadrant ¾ ring in lower left quadrant 2½ rings above horizontal row		3 rings in horizontal row 1¾ ring in upper right quadrant ¾ ring in lower left quadrant 2½ rings above horizontal row

P-25.3.2.4 Seniority criteria for selecting the parent component

The components of the fused ring system are selected and named according to P-25.3.2.1 and P-25.3.2.2. When it is necessary to locate nomenclatural features, such as indicated hydrogen or atoms with nonstandard bonding numbers, another system of locants must be used, i.e., the locants that are used to number the completed fused ring system. In these recommendations, such locants are placed outside the structure, as shown for retained names in Tables 2.2, 2.7, and 2.8. This system is fully explained and exemplified in P-25.3.3.

If there is a choice for selecting the parent component, the following criteria are considered, in order, until a decision can be made:

(a) a component containing at least one of the heteroatoms occurring earlier in the following order: N > F > Cl > Br > I > O > S > Se > Te > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl;

Examples:

azuleno[6,5-b]pyridine (PIN)
[pyridine (heterocycle) is senior to azulene (carbocycle)]

1H,18H-naphtho[1,8-rs][1,4,7,10,13,16]hexaoxacyclohenicosine (PIN)
[1,4,7,10,13,16-hexaoxacyclohenicosine (heterocycle) is senior to naphthalene (carbocycle)]

[1]benzopyrano[2,3-c]pyrrole (PIN)
(pyrrole is senior to 1-benzopyran N > O)
chromeno[2,3-c]pyrrole

2H-[1,4]dithiepino[2,3-c]furan (PIN)
(furan is senior to dithiepine; O > S)

Example:

6H-pyrazino[2,3-b]carbazole (PIN)
[carbazole (3 rings) is senior to quinoxaline (2 rings)]

Examples

2H-furo[3,2-b]pyran (PIN)
[pyran (6 ring) preferred to furan (5 ring)]

naphtho[2,3-f]azulene (PIN)
[azulene (7,5 rings) preferred to naphthalene 6,6 rings)]

Examples:

5H-pyrido[2,3-d][1,2]oxazine (PIN)
[oxazine (2 heteroatoms) preferred to pyridine (1 heteroatom)]

2H-furo[2,3-d][1,3]dioxole (PIN)
[dioxole (2 heteroatoms) preferred to furan (1 heteroatom)]

Example:

5H-[1,3]dioxolo[4,5-d][1,2]oxaphosphole (PIN)
[1,3]dioxolo[d][1,2]oxaphosphole
(an O and a P atom preferred to two O atoms )

Examples:

[1,3]selenazolo[5,4-d][1,3]thiazole (PIN)
(S,N senior to Se,N)

[1,4]oxaselenino[2,3-b][1,4]oxathiine (PIN) (O,S senior to O,Se)

Examples:

quinolino[4,3-b]acridine (PIN)
[acridine (3 rings in horizontal row)
preferred to phenanthridine (2 rings in horizontal row)]

benzo[pqr]tetraphene (PIN)
[tetraphene (3 rings in horizontal row)
preferred to chrysene or pyrene (2 rings in horizontal row)]

Example:

pyrazino[2,3-d]pyridazine (PIN)
(locants ‘1,2’ of pyridazine preferred to locants ‘1,4’ of pyrazine)

Example:

3H,5H-[1,3,2]oxathiazolo[4,5-d][1,2,3]oxathiazole (PIN)
(locants ‘1,2,3’ are lower than ‘1,3,2’)

Examples:

acephenanthyleno[5,4-k]aceanthrylene (PIN)
(the locant 2a in aceanthrylene is lower than 3a in acephenanthrylene;
see the following structures)

aceanthrylene (PIN)

acephenanthrylene (PIN)

P-25.3.2.5.1 A heteroatom common to two components must be indicated in the name of each component.

Example:

imidazo[2,1-b][1,3]thiazole (PIN)

Example:

5λ⁵-phosphinino[2,1-d]phosphinolizine (PIN)

6H-pyrazino[2,3-b]carbazole (PIN)

Fused ring systems with retained names, systematic names, or fused names are systematically numbered in the same manner. Anthracene, phenanthrene, acridine, carbazole, xanthene and its chalcogen analogues, purine, and cyclopenta[a]phenanthrene are exceptions; traditional numberings are retained. Two types of numbering are to be considered.

P-25.3.3.1 Numbering of peripheral skeletal atoms
P-25.3.3.2 Numbering of interior heteroatoms
P-25.3.3.3 Numbering of interior carbon atoms

P-25.3.3.1.1 The numbering of peripheral atoms in the preferred orientation starts from the uppermost ring. If there is more than one uppermost ring, the ring furthest to the right is chosen. Numbering starts from the nonfused atom most counterclockwise in the ring selected and proceeds in a clockwise direction around the system, including fusion heteroatoms but not fusion carbon atoms. Each fusion carbon atom is given the same number as the immediately preceding nonfusion skeletal atom, modified by a Roman letter ‘a’, ‘b’, ‘c’, ‘d’, etc.

Examples:

dipyrido[1,2-a:2′,1′-c]pyrazine (PIN)

tetracene (PIN)

benzo[m]tetraphene (PIN)

Example:

cyclopropa[de]anthracene (PIN)

In particular, the orientation and numbering of helicenes has been changed. Recommended numbering and former numbering for hexahelicene are shown below.

Higher helicenes follow the same pattern. A helicene is oriented so that a terminal ring is located in the upper right quadrant; numbering always begins in this terminal ring.

hexahelicene (PIN)
(new orientation and numbering)

Note: Orientation and numbering no longer recommended;
but still in use by CAS for the ring system having the name phenanthro[3,4-c]phenanthrene, ref. 22)

(a) low locants are assigned to heteroatoms, considered as a set without regard to the kind of heteroatom;

Examples:

cyclopenta[b]pyran (PIN)

2H,4H-[1,3]oxathiolo[5,4-b]pyrrole (PIN)

Examples:

thieno[2,3-b]furan (PIN)

1H-thieno[2,3-d]imidazole (PIN)

Examples:

	not
azulene (PIN)

Explanation: The locants ‘3a,8a’ are lower than ‘5a,8a’.

	not		nor
imidazo[1,2-b][1,2,4]triazine (PIN)

Explanation: The locant ‘4a’ is lower than ‘8a’.

Example:

	not
[1,3]diazeto[1,2-a:3,4-a′]bis([1,3]benzimidazole) (PIN)
[1,3]diazeto[1,2-a:3,4-a′]dibenzimidazole

Explanation: The locant ‘5’ is lower than ‘6’.

Example:

	not
6H-quinolizino[3,4,5,6-ija]quinoline (PIN)

Explanation: The locant ‘3a’ next to the nitrogen atom is lower than ‘5a’.

Examples:

1H-cyclopenta[l]phenanthrene (PIN)

	not
2H,4H-[1,3]dioxolo[4,5-d]imidazole		2H,6H-[1,3]dioxolo[4,5-d]imidazole
correct		incorrect

Explanation: The locant set ‘2,4’ for indicated hydrogen atoms is lower than ‘2,6’.

P-25.3.3.2.1 Interior heteroatoms that are not identified by skeletal replacement (‘a’) nomenclature are numbered after the peripheral atoms continuing the established number sequence [see also P-25.3.3.1.2 (e)]. Compare the numbering of interior carbon atoms (see P-25.3.3.3).

Example:

1H-[1,4]oxazino[3,4,5-cd]pyrrolizine (PIN)

Example:

pyrazino[2,1,6-cd:3,4,5-c′d′]dipyrrolizine (PIN)

Explanation: The heteroatom numbered ‘9’ is one bond away from ‘2a’, which is lower than ‘4b’.

Example:

phosphinolizino[4′,5′,6′:3,4,5][1,4]azaphosphinino[2,1,6-de]quinolizine (PIN)

Explanation: The nitrogen atom has a lower locant than the phosphorus atom.

P-25.3.3.3.1 Interior atoms are numbered by identifying the minimum number of bonds linking them to a peripheral atom. The locant for the interior atom is that of the peripheral atom with a superscript number corresponding to the number of bonds between the two atoms. The previous rule (Rule A-22.2 in ref. 1), which is still used in CAS index nomenclature, recommended that interior atoms follow the highest numbered peripheral atom adding Roman letters in sequence to the appropriate peripheral number.

Note: This is a major change to the rules for interior numbering of carbon atoms recommended in the 1998 publication on fusion nomenclature (see FR-5.5.2 in ref. 4).

Examples:

	not
pyrene (PIN) (recommended numbering)		Note: Former numbering, no longer recommended but still used in CAS index nomenclature.
	not
1H-phenalene (PIN) (recommended numbering)		Note: Former numbering, no longer recommended but still used in CAS index nomenclature.
	not
2H,6H-quinolizino[3,4,5,6,7-defg]acridine (PIN) (recommended numbering)		Note: Former numbering, no longer recommended but still used in CAS index nomenclature.

Example:

	not
3a2H-benzo[3,4]pentaleno[2,1,6,5-jklm]fluorene (PIN) (recommended numbering)		Note: Former numbering, no longer recommended but still used in CAS index nomenclature.

When several nonidentical components must be considered, one, and only one, can be the parent component. All other components are attached components. A component attached directly to the parent component is called a ‘first-order attached component’. A component attached to a first-order attached component is called a ‘second-order’ attached component, and so on. The parent and the first-order attached components are named as indicated for two component systems (see P-25.3.2).

P-25.3.4.1 Three types of fusion names are considered in these recommendations.

P-25.3.4.1.1 Fusion names composed of first- and higher-order attached components
P-25.3.4.1.2 Identical attached components
P-25.3.4.1.3 Multiparent names

The procedure for indicating common bond(s) between a first-order attached component and a higher-order attached component follows that for the attachment of the parent compound to the first-order attached component except that numerical locants are used instead of letters and the two sets of locants are separated by a colon. The locants of second-order attached components are primed to contrast with those of first-order attached components. The locants of third-order attached components are doubly primed, and so on.

Example:

pyrido[1′′,2′′:1′,2′]imidazo[4′,5′:5,6]pyrazino[2,3-b]phenazine (PIN)

A multiplicity of components that are identical and are all fused to a parent component or an attached component is indicated by the use of the prefix ‘di’, ‘tri’, etc. (or ‘bis’, ‘tris’, etc.). The multiplying prefix is not considered in determining the alphabetical order of attached components in names of polycomponent fusion names (see P-25.3.4.2.3). A colon separates the sets of locants, and a comma is used when letters only are present.

Examples:

difuro[3,2-b:2′,3′-e]pyridine (PIN)

5H-furo[3,2-g]dipyrano[2,3-b:3′,4′,5′-de]quinoline (PIN)
(furo before dipyrano)

Multiple occurrences of the parent component in a multiparent system is indicated by the use of multiplying prefixes, ‘di’, ‘tri’, etc, or ‘bis’, ‘tris’, etc. To distinguish between the parent components, the second has primed locants, the third double primed, etc.

The use of enclosing parentheses following ‘di’, ‘tri’, etc., indicating multiple occurrences of a parent component in these recommendations is a change from the recommendations in the 1998 publication on fusion nomenclature (ref. 4).

Examples:

cyclopenta[1,2-b:1,5-b′]bis([1,4]oxathiine) (PIN)

benzo[1,2-c:3,4-c′]bis([1,2,5]oxadiazole) (PIN)

Polycomponent fusion names are constructed by using specific orders of seniority and rules. They are elaborated as follows.

P-25.3.4.2.1 Order of seniority for selecting parent components
P-25.3.4.2.2 Order of seniority for selecting attached components
P-25.3.4.2.3 Order of citation of fusion prefixes
P-25.3.4.2.4 Order of seniority of locants (letters and numbers)

When there are two or more locations for a parent component in a fused ring structure, the following criteria are applied sequentially until a complete distinction is obtained. In the examples below, the senior location is identified by a solid box and other locations with a dashed box.

The senior location is:

(a) the location that enables the whole ring system to be named by fusion nomenclature, thus excluding names of bridged fused systems;

Example:

cyclopenta[ij]pentaleno[2,1,6-cde]azulene (PIN)
(not 1,9-methenopentaleno[1,6-ef]azulene
nor 1,9-methenodicyclopenta[cd,f]azulene;
names including the prefix ‘metheno’ are bridged fused ring names)

(b) the location that results in a name that does not require attached components higher than first-order. For didactic purposes only, names of attached components of higher order than first-order are written in bold type;

Example:

cyclopenta[h]indeno[2,1-f]naphtho[2,3-a]azulene (PIN)
(not benzo[a]benzo[5,6]indeno[2,1-f]cyclopenta[h]azulene
nor benzo[5,6]indeno[1,2-e]indeno[2,1-h]azulene;
there is no second-order component in the recommended name)

(c) the location that results in the maximum number of first-order attached components, second-order attached components, etc. This criterion results in the minimum number of higher-order attached components; hence, fewer primed fusion descriptors are required;

Example:

dibenzo[c,g]phenanthrene
(not naphtho[2,1-c]phenanthrene;
two attached components preferred to one)

(d) a location that permits the expression of the maximum number of identical attached components with multiplicative prefixes;

Example:

dinaphtho[1,2-c :2′,1′-m]picene (PIN)
(not benzo[c]phenanthreno[2,1-m]picene) (e) a location that uses a senior interparent component;

Example:

anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline (PIN)
(not phenanthro[2,1,10-def:7,8,9-d′e′f′]diisoquinoline;
anthracene is senior to phenanthrene) (f) a location that results in preferred attached components, first-order, then second-order, etc. This criterion is embellished and illustrated in the original publication (see FR-3.3.1 in ref. 4).

After selection of the parent component (see P-25.3.2.4) [or parent components and interparent component(s) if a multiparent name is chosen], other rings are identified as far as possible as attached components. If there is a choice, first-order attached components are considered first, then second-order attached components, etc. The following criteria are applied in order only until a complete decision is made:

Note: In the examples below, the preferred attached component is marked with a solid box and the alternatives with dashed boxes.

(a) if there are alternative first-order attached components, the senior ring or ring system is selected. The same procedure is applied to second-order attached components, and so on;

Example:

8H-cyclopenta[3,4]naphtho[1,2-d][1,3]oxazole (PIN)
(not 8H-benzo[6,7]indeno[5,4-d][1,3]oxazole;
naphthalene is senior to indene)

(b) the location that has lowest locants as a set for fusion of first-order attached components to the parent component;

Example:

5H-benzo[6,7]cyclohepta[4′,5′]indeno[1′,2′:3,4]fluoreno[2,1-b]furan (PIN)
(not 5H-benzo[5′,6′]indeno[1′,2′:1,2]cyclohepta[7,8]fluoreno[4,3-b]furan;
locants 1,2 are lower than 3,4 for the ‘fluoreno’ attached component)

(c) the location that has lowest locants for fusion to the parent component in order of citation.

Example:

naphtho[2′,1′:3,4]phenanthro[1,2-b]thiophene (PIN)
(not naphtho[2′,1′:3,4]phenanthro[2,1-b]thiophene,
not dibenzo[3,4:5,6]phenanthro[9,10-b]thiophene;
the locants ‘1,2’ for the ‘phenanthro’ attached component, in order of citation, are lower than ‘2,1’ or ‘9,10’)

Example:

7H-pyrrolo[2′′,1′′:1′,2′]isoquinolino[4′,3′:4,5]cyclopenta[1,2-b]acridine (PIN)
(not 7H-benzo[7′,8′]indolizino[6′,5′:4,5]cyclopenta[1,2-b]acridine;
isoquinoline is senior to indolizine)

P-25.3.4.2.3.1 Fusion between two attached components is indicated by the method described in P-25.3.2. All attached components are cited in front of the parent component. Each second-order attached component is cited in front of the first-order attached component to which it is fused, and so on to higher order attached components. If there are two or more different components, or sets of components, attached to a lower order component, they are cited in alphabetical order.

Examples:

furo[3,2-b]thieno[2,3-e]pyridine (PIN)
(furo is cited before thieno)

furo[2′,3′:4,5]pyrrolo[2,3-b]imidazo[4,5-e]pyrazine (PIN)
(furo...pyrrolo is cited before imidazo)

as-indaceno[2,3-b]-s-indaceno[1,2-e]pyridine (PIN)

difuro[3,2-b:3′,4′-e]pyridine (PIN)

5H-furo[3,2-g]dipyrano[2,3-b:3′,4′,5′-de]quinoline (PIN)
(furo is cited before pyrano)

1H-cyclopropa[b]dicyclopenta[2,3:6,7]oxepino[4,5-e]pyridine (PIN)
(not 1H-dicyclopenta[2,3:6,7]oxepino[4,5-b]cyclopropa[e]pyridine;
cyclopropa is cited before dicyclopentaoxepino, which is treated as one component)

Example:

4H,16H,20H,26H-cyclopenta[4,5]oxepino[3,2-a]bis(cyclopenta[5,6]oxepino)[3′,2′-c:2′′,3′′-h]cyclopenta[6′′′,7′′′]oxepino[2′′′,3′′′-j]phenazine (PIN)

Example:

2H-[1,2]oxazolo[5,4-c][1,3]oxazolo[3,2-a]pyridine (PIN)

If there is a choice of locants, letters, or numerals (consistent with the numbering of the component), the lower letters or numbers are selected in accordance with the following criteria, which are considered in order until a complete decision can be made:

(a) parent component letters as a set;

Example:

furo[3,2-h]pyrrolo[3,4-a]carbazole (PIN)
(not furo[2,3-b]pyrrolo[3,4-i]carbazole;
a,h is lower than b,i)

Examples:

furo[3,4-b]thieno[2,3-e]pyrazine (PIN)
(not furo[3,4-e]thieno[2,3-b]pyrazine;
b...e is lower than e...b)

2H,10H-dipyrano[4,3-b:2′,3′-d]pyridine (PIN)
(not 2H,10H-dipyrano[2,3-d:4′,3′-b]pyridine;
b...d is lower than d...b)

diindeno[1,2-i:6′.7′,1′-mna]anthracene (PIN)
(not diindeno[6,7,1-mna:1′,2′-i]anthracene;
i...mna is lower than mna...i)

Examples:

10H-furo[3′,2′:5,6]pyrido[3,4-a]carbazole (PIN)
(not 10H-furo[2′,3′:2,3]pyrido[5,4-a]carbazole;
the locant set ‘3,4’ is lower than ‘4,5’ for attaching pyrido to carbazole)

cyclopenta[1,2-b:5,1-b′]difuran (PIN, a multiparent name)
(not cyclopenta[1,2-b:2,3-b′]difuran;
the locant set ‘1,1,2,5’ is lower than ‘1,2,2,3’)

Example:

pyrazolo[4′,3′:6,7]oxepino[4,5-b]indole (PIN)
(not pyrazolo[3′,4′:2,3]oxepino[5,4-b]indole;
the locant set ‘4,5’ is lower than ‘5,4’)

Example:

pyrano[3′,2′:4,5]cyclohepta[1,2-d]oxepine (PIN)
(not pyrano[2′,3′:5,6]cyclohepta[1,2-d]oxepine;
the locant set ‘4,5’ is lower than ‘5,6’)

Example:

pyrrolo[3′,2′:4,5]cyclohepta[1,2-b]quinoline (PIN)
(not pyrrolo[2′,3′:5,4]cyclohepta[1,2-b]quinoline;
the locant set ‘4,5’ is lower than ‘5,4’)

Example:

7H-indeno[7′,1′:5,6,7]cycloocta[1,2,3-de]quinoline (PIN)
(not 7H-indeno[3′,4′:5,6,7]cycloocta[1,2,3-de]quinoline;
the locant set ‘1′,7′’ is lower than ‘3′,4′’)

Example:

pyrano[2′′,3′′:6′,7′]thiepino[4′,5′:4,5]furo[3,2-c]pyrazole (PIN)
(not pyrano[3′′,2′′:2′,3′]thiepino[5′,4′:4,5]furo[3,2-c]pyrazole;
the locant set ‘4′,5′’ is lower than ‘5′,4′’)

Heterobicyclic compounds consisting of a heteromonocycle fused to a benzene ring in which the benzene ring is not part of a system having a retained name such as quinoline or naphthalene are treated as a single component unit (a ‘benzoheterocycle’, see P-25.2.2.4). They may be treated as a parent component or an attached component depending on the order of seniority described in P-25.3.2.4. However, this approach is not used if it disrupts a multiparent system (see P-25.3.5.3, below) or the use of multiplicative prefixes (see P-25.3.6.1, below).

P-25.3.5.1 A benzoheterocycle as a parent component

Example:

thieno[3,2-f][2,1]benzothiazole (PIN)
(2,1-benzothiazole is senior to 1-benzothiophene)

Example:

[1,2]benzoxazolo[6,5-g]quinoline (PIN)
(quinoline is senior to 1,2-benzoxazole)

Example:

benzo[1,2-b:4,5-c′]difuran (PIN)
(not furo[3,4-f][1]benzofuran;
a multiparent name is preferred to a two-component fused name)

Examples:

4H-[1,4]thiazino[2,3-g]quinoline (PIN)
(the retained name quinoline must be used)

10H-furo[3′,2′:4,5]indeno[2,1-b]pyridine (PIN)
(not 10H-[1]benzofuro[5′,4′:3,4]cyclopenta[1,2-b]pyridine;
pyridine is senior to furan; indene, as a retained name, must be used)

6H-dibenzo[b,d]pyran (PIN)
(not 6H-benzo[c][1]benzopyran;
not 6H-benzo[c][2]benzopyran)
6H-benzo[c]chromene

P-25.3.6.1 Two or more components that are identical and both fused to a parent component are indicated by the use of multiplying prefixes (‘di’, ‘tri’, etc. or ‘bis’, ‘tris’, etc.). If a complete set of locants is used for first-order attached components fused to the parent component, they are cited together separated by a colon. If abbreviated sets of locants are used, the letters are separated by a comma. If complete sets of locants are used for second-order attached components fused to a first-order attached component, or for higher-order cases, the locants are cited together separated by a semicolon. If abbreviated sets are used, they are separated by a colon. To distinguish between two or more components of the same order, the locants of the second component are primed (or double primed if the first is primed, etc.), the third double primed, and so on.

Examples:

difuro[3,2-b:3′,4′-e]pyridine (PIN)

dibenzo[c,e]oxepine (PIN)

9H-dibenzo[g,p][1,3,6,9,12,15,18]heptaoxacycloicosine (PIN)

dibenzo[4,5:6,7]cycloocta[1,2-c]furan (PIN)

dithieno[2′,3′:3,4;2′′,3′′:6,7]cyclohepta[1,2-d]imidazole (PIN)

cyclopenta[b]dibenzo[3,4:6,7]cyclohepta[1,2-e]pyridine (PIN)

2H,9H-bis([1,3]benzodioxolo)[4,5,6-cd:5′,6′-f]indole (PIN)

Fusion of a higher-order attached component to a system named with a multiplicative prefix requires each set of attached components to be specified separately.

Example:

furo[3,4-b]furo[3′,2′:4,5]furo[2,3-e]pyridine (PIN)

Two or more groups of identical components (including identical fusion locants between these components) fused to another component, are indicated by the use of the multiplying prefixes ‘bis’, ‘tris’, etc., and the group of components is cited within parentheses.

Example:

bis(pyrimido[5′,4′:4,5]pyrrolo)[2,3-c:3′,2′-e]pyridazine (PIN)

Multiple occurrences of the parent component in a multiparent system is indicated by the use of multiplying prefixes, ‘di’, ‘tri’, etc., or ‘bis’, ‘tris’, etc. To distinguish between the parent components, the second has primed locants, the third double primed, etc.

Two or more nonoverlapping locations for parent components that are ortho- or ortho- and peri-fused to the same first-order interparent component are treated as a multiparent system and given a multiparent name. Similarly a system with three, five, seven, etc. interparent components is treated as an extended multiparent system. Each pair of second- or higher-order interparent components must be identical.

P-25.3.7.1 Multiparent systems with one interparent component

Multiple occurrences of the parent component in a multiparent system are indicated by the use of a multiplying prefix (‘di’, ‘tri’, etc., or ‘bis’, ‘tris’, etc.). To distinguish between the parent components, the second parent component has primed letters, the third double primed, etc.; the sets of locants are separated by a colon.

The use of enclosing parentheses following ‘bis’, ‘tris’, etc. indicating multiple occurrences of a parent component in these recommendations is a change from the recommendations in the 1998 publication on fusion nomenclature (ref. 4).

Examples:

benzo[1,2-f:4,5-g′]diindole (PIN)

benzo[1,2:4,5]di[7]annulene (PIN)

1H-benzo[1,2:3,4:5,6]tri[7]annulene (PIN)

[1,4,7,10]tetraoxacyclohexadecino[13,12-b:14,15-b′]dipyridine (PIN)

cyclopenta[1,2-b:5,1-b′]bis([1,4]oxathiine) (PIN)

phenanthro[4,5-bcd:1,2-c′]difuran (PIN)

cyclopenta[1,2-b:1,5-b′]bis([1,4]oxathiine) (PIN)

benzo[1,2-c:3,4-c′]bis([1,2,5]oxadiazole) (PIN)

furo[3,2-b:4,5-b′]difuran (PIN, a multiparent name)
[not difuro[3,2-b:2′,3′-d]furan (a multiplicative prefix name);
the multiparent method applies to situations in which there are three identical fused rings or ring systems
according to the principle that a maximum of parent components is preferred to a maximum of attached components]

Additional attached components may be fused to any of the components of a multiparent system. If the choice of locants described in P-25.3.4.2.4 does not permit a final choice, seniority is given to the unprimed component and the fusion letters are assigned with the lower letter used for fusion to the connecting component. Great care is needed with the use of primes, double primes, etc. to ensure that there is no ambiguity. Thus additional components fused to the interparent component(s) are cited in front of the prefix for the interparent component.

Examples:

tribenzo[c,d′,e]benzo[1,2-a:4,5-a′]di[7]annulene (PIN)

thieno[2′,3′:3,4]cyclopenta[1,2-e]furo[3′,4′:6,7]cyclohepta[1,2-b:5,4-b′]dipyridine (PIN)
(the furan ring is fused to the interparent component; thus, alphabetical order does not apply)

When two (or more) possible parent components are separated by an odd number of interparent components and these are ordered symmetrically with respect to their component rings (but not necessarily with their fusion locants), the whole system is treated as a multiparent system. Names are formed by extending P-25.3.7.1 in two ways as follows:

(a) Second- and higher-order interparent components are named using the multiplying prefixes ‘di’, ‘tri’, etc., or ‘bis’, ‘tris’, etc. Appropriate locants are assigned to interparent components, unprimed and primed for first-order interparent components, double primed for second-order interparent components, tripled primed for third-order interparent components, and so on.

(b) When symmetry permits grouping of interparent components and parent components, such groups can be formed and cited as such using the multiplying prefixes ‘bis’, ‘tris’, etc. to denote groups that are enclosed within parentheses. Unprimed locants only are used within such groupings.

Examples:

benzo[1′′,2′′:3,4;4′′,5′′:3′,4′]dicyclobuta[1,2-b:1′,2′-c′]difuran (PIN)

benzo[1′′,2′′:3,4;4′′,5′′:3′,4′]bis(cyclobuta[1,2-c]furan)

P-25.3.8.1 In preferred IUPAC names and in general nomenclature, numerical and/or letter locants are omitted in fused ring systems with only first-order monocyclic attached hydrocarbon components, ‘benzo’, and those described in P-25.3.2.2.1; the omission is also recommended when the fused ring system is made of two monocyclic hydrocarbons.

Examples:

benzo[a]tetracene (PIN)

1H-cyclopenta[8]annulene (PIN)

cyclopropa[de]anthracene (PIN)

benzo[g]quinoline (PIN)

1H-naphtho[2,3-d][1,2,3]triazole (PIN)
(not 1H-naphtho[2,3][1,2,3]triazole)

Example:

cyclopenta[4,5]pyrrolo[2,3-c]pyridine (PIN)

Example:

furo[3′,4′:5,6]pyrazino[2,3-c]pyridazine (PIN)

Examples:

naphtho[2,1,8-def]quinoline (PIN)

quinolizino[4,5,6-bc]quinazoline (PIN)

Example:

naphtho[1,8a-b]azirine (PIN)

This Section is based on the publication ‘Nomenclature of Fused and Bridged Fused Ring Systems’ (ref. 4). Sections P-25.4.2.1.2, P-25.4.2.1.4 and P-25.4.2.2.1 contain modifications to Section FR-8.3 of the 1998 publication (ref. 4). No other modifications have been made to the 1998 publication.

P-25.4.1 Definitions and terminology
P-25.4.2 Names of bridges
P-25.4.3 Naming bridged fused ring systems
P-25.4.4 Numbering bridge atoms
P-25.4.5 Order for numbering of bridges

P-25.4.1.1 Bridged fused ring system. A ring system in which some of the rings constitute a fused ring system (see P-25.0 through P-25.3) and the remaining rings are created by one or more bridges.

P-25.4.1.2 Bridge. An atom or group of atoms is named as a bridge by means of a prefix if it fulfills one or more of the following:

(a) if it connects two or more nonadjacent positions of the same ring in a fused ring system;

(b) if it connects two or more positions of different rings of a fused ring system and does not thereby form a new ortho- and peri- fused ring;

(c) if it connects positions of a ring of a fused ring system to a previously described bridge but cannot be included as part of that bridge;

(d) if it connects the atoms at the end of a bond common to two rings of a fused ring system;

(e) if it is used to describe a system with only ortho- or ortho- and peri- fusions that cannot be named entirely by fusion principles.

criterion (a)

criterion (b)

criterion (c)

criterion (d)

criterion (e)

P-25.4.1.4 Simple bridge A bridge that describes an atom, or groups of atoms, which may be described as a single unit, for example ‘epoxy’, ‘butano’, ‘benzeno’.

P-25.4.1.5 Composite bridge. A group of atoms that can only be described as a contiguous sequence of simple bridges, for example (epoxymethano) = epoxy + methano = –O-CH₂–.

P-25.4.1.6 Divalent bridge. A bridge that is connected by single bonds to two different positions of a fused ring system or bridged fused ring system. All bridges described in P-25.4.2.1 are simple bridges.

P-25.4.1.7 Polyvalent bridge. A bridge that is connected to a fused ring system by three or more single bonds or their multiple bond equivalent. Polyvalent bridges may often be described by a combination of two simple divalent bridges. Polyvalent bridges may be further classified as bipodal, tripodal, etc., when the bridge is attached to two, three, or more positions of the fused ring system.

Examples:

a tripodal trivalent bridge

a bipodal trivalent bridge

P-25.4.1.9 Dependent bridge. A bridge that connects one or more positions of a fused ring system to one or more positions of a simple or composite independent bridge, and cannot be expressed as part of a larger composite bridge.

Example:

4,5,12-epimethanetriyl-2,9,7-epipropane[1,2,3]triylanthracene (PIN)
(the epimethanetriyl bridge C-14 is a dependent bridge;
the epipropane[1,2,3]triyl bridge at C-11 to C-13 is an independent bridge;
see P-25.4.3.2 for attachment locants)

The extensive list of bridge names given in the publication on fused ring and bridged fused ring nomenclature (ref. 4) has been thoroughly reviewed and updated in the context of the recommendations given herein. Most of the changes occur in the names of acyclic heteroatom bridge names, for which see P-25.4.2.1.4 and P-25.4.2.2.1. For the use of skeletal replacement (‘a’) nomenclature for naming bridges, see P-25.5.1.2.

P-25.4.2.1 Divalent bridges.

P-25.4.2.1.1 A divalent acyclic hydrocarbon bridge is named as a prefix derived from the corresponding unbranched hydrocarbon name by changing the final letter ‘e’ to ‘o’. The locant for a double bond, if present, is indicated in square brackets between the hydrocarbon prefix and the ending ‘eno’; this locant is not the locant used in the final numbering of the bridged fused ring system (see P-25.4.4). If there is a choice, low numbers are preferred (e.g., prop[1]eno rather than prop[2]eno).

Examples:

–CH2–	methano (preferred prefix)
–CH2-CH2–	ethano (preferred prefix)
–CH2-CH2-CH2–	propano (preferred prefix)
–CH=CH–	etheno (preferred prefix)
	prop[1]eno (preferred prefix)
	but[1]eno (preferred prefix)
	but[2]eno (preferred prefix)
	buta[1,3]dieno (preferred prefix)

P-25.4.2.1.2 A divalent monocyclic hydrocarbon bridge other than benzene is named by the same prefix as that used as a fusion prefix (P-25.3.2.2). To distinguish between these two, ‘epi’ is added in front of the name when used as a bridge prefix; the letter ‘i’ in the prefix ‘epi’ is elided before the letters ‘i’ and ‘o’ of the following term. [CAS uses the italicized prefix ‘endo’.] The bridge is assumed to have the maximum number of noncumulative double bonds consistent with its attachments to the fused ring system or to other bridges. The positions of the free valences of the bridge are indicated by locants in square brackets cited directly in front of the bridge name; these locants are not those used for bridge atoms in the final structure, for which see P-25.4.4. Locants for indicated hydrogen atoms, if present, are those for the final structure and are cited in front of the completed name (see P-25.4.3.4).

Examples:

	[1,3]epicyclopropa (preferred prefix) [‘epicyclopropa’ in ref. 4, FR 8.3.1(b)]
	[1,2]epicyclopenta (preferred prefix)

Examples:

[1,2]benzeno (preferred prefix)
(not [1,2]epibenzo;
benzo is the name of a fusion prefix)

[1,3]benzeno (preferred prefix)
(not [1,3]epibenzo;
benzo is the name of a fusion prefix)

[1,2]naphthaleno (preferred prefix)
(not [1,2]epinaphtho;
naphtho is the name of a fusion prefix)

[1,3]epindeno (preferred prefix)
(not [1,3]indeno;
indeno is the name of a fusion prefix)

The following names are now recommended as preselected (see P-12.2) bridge prefixes: ‘sulfano’, –S–;‘disulfano’, –SS–; ‘selano’, –Se–; ‘tellano’, –Te–; ‘azano‘, –NH–; ‘diazeno’, –N=N–; ‘epitriazano’, –NH-NH-NH–; and –NH-N=N–, ‘epitriaz[1]eno’. The bridge prefixes ‘epithio’, ‘epidithio’, ‘episeleno’, ‘epitelluro’, and ‘epimino’ [ref. 4, FR-8.3.1(d)] may be used in general nomenclature, as noted in the following examples.

Examples:

–O–	epoxy (preselected prefix) (not epoxidano)
–O-O–	epidioxy (preselected prefix) (not epiperoxy)
–O-O-O–	epitrioxy [preselected prefix, see ref. 4, FR-8.3.1(d)] epitrioxidanediyl
–S–	sulfano (preselected prefix) epithio [see ref. 4, FR-8.3.1(d)]
–SH2–	λ4-sulfano (preselected prefix)
–S-S–	disulfano (preselected prefix) epidithio [see ref. 4, FR-8.3.1(d)]
–Se–	selano (preselected prefix) episeleno [see ref. 4, FR-8.3.1(d)]
–Te–	tellano (preselected prefix) epitelluro [see ref. 4, FR-8.3.1(d)]
–SiH2–	silano (preselected prefix)
–SnH2–	stannano (preselected prefix)
–NH–	azano (preselected prefix) epimino [see ref. 4, FR-8.3.1(d)] (not imino given in ref. 1, C-815.2)
–NH-NH–	diazano (preselected prefix) biimino (see ref. 1, B-15.1)
–N=N–	diazeno [preselected prefix; see ref. 4 FR-8.3.1(d)]) azo (see ref. 1, B-15.1)
–NH-NH-NH–	epitriazano (preselected prefix) (not triazano, which has been used for H2N-NH-NH–, see C-942.3, ref. 1, which is triazan-1-yl in these recommendations, see P-68.3.1.4.1)
	epitriaz[1]eno (preselected prefix) (not triaz[1]eno, which has been used for H2N-N=N–, see C-942.3, ref. 1, which is triaz-1-en-1-yl in these recommendations, see P-68.3.1.4.1) azimino (see ref. 1, B-15.1)
–PH–	phosphano (preselected prefix)
–BH–	borano (preselected prefix)

Examples:

epoxireno (preferred prefix)

[2,3]furano (preferred prefix)
(not [2,3]epifurano;
nor [2,3]epifuro)

[2,3]epipyrano (preferred prefix)

[3,4]epi[1,2,4]triazolo (preferred prefix)

[2,5]epipyrrolo (preferred prefix)

P-25.4.2.2.1 A polyvalent bridge consisting of one atom (other than hydrogen) is named as a prefix based either on a substitutive prefix to which the prefix ‘epi’ is added as described in P-25.4.2.1.4 or by changing the ending ‘ano’ in the name of a divalent monoatomic bridge described in P-25.4.2.1.4 into ‘eno’, for example ‘metheno’ from ‘methano’. Polyvalent bridges are enclosed within parentheses in names of bridged fused systems; as a reminder, parentheses are placed around the names of the bridges themselves in the examples below. Where there is a choice, an attachment by a single bond, i.e., ‘yl’, is to the lower locant of a parent ring system. For ‘epi’, see P-25.4.2.1.4.

Examples:

–CH=	(metheno) (preferred prefix) (epimethanylylidene)
	(epimethanetriyl) (preferred prefix) (methyno)
	(epimethanediylylidene) (preferred prefix)
–N=	(azeno) (preselected prefix) (epiazanylylidene) (not ‘nitrilo’, see ref. 2, R-9.2.2)
	(episilanetetrayl) (preselected prefix)
	(epinitrilo) (preselected prefix) epiazanetriyl [ref. 4, FR-8.3.2 (a)] [not nitrilo]
–P=	(phospheno) (preselected prefix) (epiphosphanylylidene) [ref. 4, FR-8.3.2 (a)]
	(epiphosphanetriyl) (preselected prefix) [not phosphanetriyl, [ref. 4 FR-8.3.2 (a); nor phosphinidyne]

Examples:

–CH2-CH=	(epiethanylylidene) (preferred prefix)
	(epiethane[1,1,2]triyl) (preferred prefix)
	(epiethene[1,1,2]triyl) (preferred prefix)
	(epibuta[1,3]diene[1,1,4]triyl) (preferred prefix)
	(epibut[3]ene[1,1,2,4]tetrayl) (preferred prefix)
=N-N=	(epidiazanediylidene) (preselected prefix)
	(epibenzene[1,2,3,4]tetrayl) (preferred prefix)

P-25.4.2.3.1 Composite bridge names are formed by concatenating the names of two or more simple bridges. Unless cited first, the prefix ‘epi’ (or ‘ep’ before the letter ‘i’ or ‘o’ of the following term) is omitted. The prefixes are cited without elision in order starting from the senior prefix based on the following criteria applied in turn until a decision is reached.

(a) the simple bridge containing the heteroatom appearing first in the list: O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl;

(b) the simple bridge containing the senior ring system;

(c) the simple bridge containing the longer acyclic chain;

(d) the simple bridge that occurs first in alphanumerical order.

Examples:

–O-CH2–	(epoxymethano) (preferred prefix)
–NH-CH2-CH2–	(azanoethano) (preferred prefix)
–O-S-O–	(epoxysulfanooxy) (preselected prefix)
	(epoxyethane[1,1,2]triyl) (preferred prefix)
–O-CH2-CH<	(epoxyethane[1,2,2]triyl) (preferred prefix)
	([1,4]benzenomethano) (preferred prefix)
	(epoxy[1,4]benzeno) (preferred prefix)
	([2,3]furanomethano) (preferred prefix)
	([3,2]furanomethano) (preferred prefix)
	[ethano([2,5]epipyrrolo)methano] (preferred prefix)
–CH2-O-CH=	(methanooxymetheno) (preferred prefix)

P-25.4.3 Naming bridged fused ring systems

P-25.4.3.1 Citation of bridges
P-25.4.3.2 Attachment locants
P-25.4.3.3 Choice of attachment locants
P-25.4.3.4 Selection of bridges

If there is more than one bridge, they are cited in alphabetical order, unless one bridge is dependent on another. In that case, the dependent bridge is cited in front of all independent bridges. Two or more identical bridges are indicated by the numerical prefixes ‘di’, ‘tri’, etc. with simple bridges (P-25.4.1.4), or ‘bis’, ‘tris’, etc. with composite bridges (P-25.4.1.5) or if ‘di’, ‘tri’, etc. would be ambiguous. The locant pairs of identical bridges are separated by colons.

P-25.4.3.2 Attachment locants

The fused ring system to be bridged is numbered in the usual way. For selection of the fused ring to be bridged see P-25.4.3.4.2.

P-25.4.3.2.1 Divalent symmetric bridges

The locants of the positions on the fused ring system to which divalent symmetric bridges (are) attached are cited in numerical order in front of each bridge name.

Examples:

9,10-ethanoanthracene (PIN)

1,4:6,9-dimethanooxanthrene (PIN)
1,4:6,9-dimethanodibenzo[b,e][1,4]dioxine

6,9-epoxy-1,4-methanobenzo[8]annulene (PIN)

1H-1,4-ethanothioxanthene (PIN)

5,12:6,11-di[1,2]benzenodibenzo[a,e][8]annulene (PIN)

Locants of the positions on the fused ring system to which polyvalent or composite bridges are attached are cited in the order expressed or implied by the name of the bridge. Methods of assigning locants for the free valences of bridges are given in section P-25.4.2. In the absence of specific locants, a single free valence (yl) is assigned a lower locant than a double free valence (ylidene). If there is a choice, locants are cited in numerical order. Indicated hydrogen atoms are cited at the front of the complete bridged fused ring name and not in front of names of the bridges themselves (P-25.7.1).

Examples:

2H-3,5-(epoxymethano)furo[3,4-b]pyran (I) (PIN)
(not 7H-3,5-(epoxymethano)furo[2,3-c]pyran) (II)
[preferred ring system, see P-25.4.3.4.2 (d)]

7H-5,3-(epoxymethano)furo[2,3-c]pyran (I) (PIN)
(not 4H-6,1-(epoxymethano)furo[3,4-c]pyran (II)
[preferred ring system, see P-25.4.3.4.2 (d)]

10,5-[2,3]furanobenzo[g]quinoline (PIN)

1,4:8,5-bis(epoxymethano)anthracene (PIN)

If there is a choice of attachment locants after the application of P-25.4.3.2, seniority is established in the following order:

(a) the lowest set of locants for all the bridge attachment points considered as a set;

Examples:

1,4-epoxynaphthalene (PIN)
(not 5,8-epoxynaphthalene)

5,8-epoxy-1,3-methanoanthracene (PIN)
(not 1,4-epoxy-5,7-methanoanthracene;
the locant set 1,3,5,8 is lower than 1,4,5,7)

Examples:

1,4-ethano-5,8-methanoanthracene (PIN)

1,4-epoxy-5,8-methanonaphthalene (PIN)

When a polycyclic ring system cannot be named completely as a fused ring system, possible ways for naming it as a bridged fused system are considered. Bridges are selected so that a recommended fused ring system as described in P-25.1 through P-25.3 is the parent fused ring system that is bridged.

P-25.4.3.4.1 Naming of ortho- or ortho- and peri-fused systems

The ring system that remains after removal of the bridge(s) is named according to P-25.1 through P-25.3. The maximum number of noncumulative double bonds is assigned to the fused ring system after the insertion of the bridge. Thus, in allowing for the bonds of the bridge, the number of noncumulative double bonds and/or the need for indicated hydrogen may be different than in the parent ring system. If needed, indicated hydrogen is used to identify the specific isomer and is cited in front of the completed bridged fused ring name (see P-25.7.1.3.2).

Examples:

4a,8a-ethanonaphthalene (PIN)

9H-9,10-ethanoacridine (PIN)

1,5-methanoindole (PIN)

9H-9,10-(epiethanylylidene)anthracene
[not 9H-10,9-(epiethanylylidene)anthracene;
the ‘yl’ suffix is given preference for the low locant
of attachment to the ring system (see P-25.4.3.2.2)]

If there is a choice for selecting the fused ring system to be bridged, the criteria in the following list are applied in order until a complete decision is reached. Two structures might differ in the distribution of double bonds as described in P-25.4.3.4.1. Locants of the fused ring system where bridges are attached are then cited in front of the name of the bridge and in the same order as the locants for the free valences of the bridge (see P-25.4.3.2.2). The fused ring system, before bridging must:

(a) contain the maximum number of rings;

Examples:

	not
(I)		(II)
1H-1,3-propanocyclobuta[a]indene (I) (PIN) [not 8,10,1-(epiethane[1,1,2]triyl)benzo[8]annulene(II) (for order of ring attachment locants see P-25.4.3.2.2)

Explanation: the correct name has three rings in the fused ring system; the incorrect name has only two rings in the fused ring system.

Example:

(I)	(II)	(III)
6,7-(epiprop[1]en[1]yl[3]ylidene)benzo[a]cyclohepta[e][8]annulene (I) (PIN) [not 7,6-(epiprop[1]en[1]yl[3]ylidene)benzo[a]cyclohepta[e][8]annulene (II); the ‘yl’ suffix is given the lower attachment point of the ring system (see P-25.4.3.2.2) [not 4,5-buta[1,3]dienodibenzo[a,d][8]annulene (III); the fused ring system benzo[a]cyclohepta[e][8]annulene has 17 atoms whereas the fused ring system dibenzo[a,d][8]annulene has only 16 atoms]

(c) have the fewer heteroatoms in the fused ring system before bridging;

Example:

	not
(I)		(II)
1,4:8,5-bis(epoxymethano)anthracene (I) (PIN)		[not 1H,3H-1,4:6,9-bisethenobenzo[1,2-c:5,4-c′]dipyran (II);
zero heteroatoms in the fused ring system before bridging is fewer than two]

(d) consist of the most senior ring system, when the seniority order is applied to the parent fused ring system (see P-44.2);

Examples:

	not
(I)		(II)
1,7-ethano[4,1,2]benzoxadiazine (I) (PIN)		[not 4,6-ethanopyrido[1,2-d][1,3,4]oxadiazine (II);
the locant set ‘1,2,4’ for the heteroatoms in (I) is lower than the locant set ‘1,3,9’ in (II) (see P-14.3.5)]

(I)	(II)
1,12-ethenobenzo[4,5]cyclohepta[1,2,3-de]naphthalene (I) (PIN) {not 1,12-ethenobenzo[c]phenanthrene (II); the ring size set ‘7,6,6,6’ in (I) is preferred to the ring size set ‘6,6,6,6’ in (II) [see P-25.3.2.4 (c) and P-44.2.2.2.3 (a)]}

	not
(I)		(II)
octadecahydro-6,13-methano-2,14:7,9-dipropanodicyclohepta[a,e][8]annulene (I) (PIN) {not octadecahydro-7,14-methano-4,6:8,10-dipropanodicyclohepta[a,d][8]annulene (II); in the correct name a greater number of rings of the fusion ring system to be bridged are in a horizontal row, ‘3’ versus ‘2’ [see P-25.3.2.3.2 (a) and P-44.2.2.2.3 (b)]} hexacyclo[15.3.2.23,7.12,12.013,21.011,25]pentacosane (see P-23.2.6.3)

(e) have the minimum number of polyvalent bridges;

Examples:

	not
(I)		(II)
2,6:5,7-dimethanoindeno[7,1-bc]furan (I) (PIN) (for the order of ring attachment locants see P-25.4.3.2.2) [not 5,7,2-(epiethane[1,1,2]triyl)indeno[7,1-bc]furan (II); the PIN (I) has no polyvalent bridges; the incorrect name (II) has one polyvalent bridge]

	not
(I)		(II)
6,12-epoxy-5,13-methanobenzo[4,5]cyclohepta[1,2-f][2]benzopyran (I) (PIN) 6,12-epoxy-5,13-methanobenzo[4,5]cyclohepta[1,2-f]isochromene (I) (for the order of ring attachment locants see P-25.4.3.2.2) [not 7,5,13-(epoxymethanetriyl)benzo[4,5]cyclohepta[1,2-f]isochromene (II); the PIN (I) has no polyvalent bridges; the incorrect name (II) has one polyvalent bridge]

(f) have the minimum number of dependent bridges;

Example:

	not
(I)		(II)
6,11-buta[1,3]dieno-3,8-phosphano[1,4]diazocino[2,3-g]cinnoline (I) (PIN) [not 3,15-phosphano-6,11-buta[1,3]dieno[1,4]diazocino[2,3-g]cinnoline (II); the PIN (I) has no dependent bridges; the incorrect name (II) has one dependent bridge]

(g) have the minimum number of atoms in dependent bridges;

Example:

6,17-methano-10,13-pentanonaphtho[2,3-c][1]benzazocine (PIN)
[not 13,17-ethano-6,10-butanonaphtho[2,3-c][1]benzazocine)
nor 10,17-ethano-6,13-butanonaphtho[2,3-c][1]benzazocine);
the PIN has only one atom in a dependent bridge]

	not
(I)		(II)
8,7-(azenoepietheno)cyclohepta[4,5]cycloocta[1,2-b]pyridine (I) (PIN) [not 8,7-(azenoepiethanediylidene)cyclohepta[4,5]cycloocta[1,2-b]pyridine (II); the PIN has a composite bridge consisting of a trivalent bridge and a divalent bridge whereas the incorrect name contains a composite bridge consisting of a trivalent bridge and a tetravalent bridge]

(i) have the lowest locants at the location of bridges, first for independent bridges then dependent bridges;

Example:

14,5-(metheno)-3,2,4-(epiprop[2]en[1,3]diyl[1]ylidene)dicyclopenta[f,f′]pentaleno[1,2-a:6,5-a′]dipentalene (PIN)
the locants set for the independent bridge ‘3,2,4’ is the lowest; the locant for single bond attachment of metheno bridge is cited first.
[not 5,14-(metheno)-3,4,2-(epipropan[1]yl[1,3]diylidene)dicyclopenta[f,f′]]pentaleno[1,2-a:6,5-a′]dipentalene;
not 14,5-(metheno)-4,2,3-(epiprop[2]en[1,3]diyl[1]ylidene)dicyclopenta[f,f′]]pentaleno[1,2-a:6,5-a′]dipentalene;
not 5,14-(metheno)-4,3,2-(epipropan[1]yl[1,3]diylidene)dicyclopenta[f,f′]]pentaleno[1,2-a:6,5-a′]dipentalene;
(the locants ‘3,2,4’ for independent bridge is lower than ‘3,4,2’, ‘4,2,3’ and ‘4,3,2’]

Example:

1,4-dihydro-1,4-ethanoanthracene (PIN)
(not 1,2,3,4-tetrahydro-1,4-ethenoanthracene)

Bridge atoms are numbered continuing from the highest locant of the fused ring system. If there is more than one bridge atom (excluding hydrogen) the numbering starts at the end of the chain or ring atom connected to the bridgehead of the fused ring system having the highest locant. With composite bridges, each component is completely numbered before the next component. Fusion atoms in fused ring bridges are numbered in accordance with P-25.3.3.1.1.

Example:

1,4-ethanonaphthalene (PIN)

(a) low locants for heteroatoms;

(b) low locants for bridgehead atoms within a bridge;

Examples:

9,10-[1,2]benzenoanthracene (PIN)

10,5-[2,3]furanobenzo[g]quinoline (PIN)

12H-5,10-[2,5]epipyranobenzo[g]quinoline (PIN)

6,13-(methano[1,2]benzenomethano)pentacene (PIN)

6b,12b-[1,8]naphthalenoacenaphthyleno[1,2-a]acenaphthylene (PIN)
Note: Locants 13a and 17a are generated in the same way as fusion locants for nonbridged fused ring systems, see P-25.3.3.1.1.

P-25.4.5.1 Independent bridges are numbered before dependent bridges.

Example:

13,16-epoxy-1,4:5,8-diepoxy-9,10-[1,2]benzenoanthracene (PIN)
Note: Dependent bridges are cited before independent bridges but are numbered after them, see P-25.4.3.1.

Examples

1,4-ethano-5,8-methanoanthracene (PIN)

6,14:7,14-dimethanobenzo[7,8]cycloundeca[1,2-b]pyridine (PIN)

Example:

6,13-ethano-6,13-methanodibenzo[b,g][1,6]diazecine (PIN)

The fusion principles described in P-25.1 through P-25.3 apply only between pairs of components. It is not possible by these principles to name a system in which a third component is ortho- and peri-fused to two components that are themselves ortho- or ortho- and peri-fused together. It is important to recall that benzo heterocycles are considered as one component, thus permitting the construction of names that would not otherwise be possible.

2H-[1,3]benzodioxino[6′,5′,4′:10,5,6]anthra[2,3-b]azepine (PIN)
Note: A normal fusion name is not possible when the four components
azepine, anthracene, 1,3-dioxine and benzene are treated individually;
therefore, the use of a benzo name component is necessary;
1-benzazepine cannot be the parent ring because this
would require breaking of the attached component having a retained name, anthra,
which is not allowed, see P-25.3.5.

P-25.5.1 Skeletal replacement (‘a’) nomenclature

P-25.5.1.1 If the corresponding hydrocarbon fused ring system can be named by fusion principles or has a retained name, then heteroatoms are identified by skeletal replacement (‘a’) nomenclature using the appropriate ‘a’ prefixes (see P-22.2.3). The numbering of the fused hydrocarbon system is not altered by the ‘a’ prefixes.

When the fusion principles discussed in P-25.1 through P-25.3 do apply, no skeletal replacement (‘a’) name is recommended. The procedure is valid only for cases described in P-25.5.

Examples:

1,2,3,4,5,6-hexaazacyclopenta[cd]pentalene (PIN)

1,3a¹,4,9-tetraazaphenalene (PIN)

5H,12H-2,3,4a,7a,9,10,11a,14a-octaazadicyclopenta[ij:i′j′]benzo[1,2-f:4,5-f′]diazulene (PIN)

Examples:

2,3,9-trioxa-5,8-methanocyclopenta[cd]azulene (PIN)

1H-3,10-dioxa-2a¹,5-ethanocycloocta[cd]pentalene (PIN)

2H,5H-4,6,11-trioxa-1-thia-5,8b-[1,2]epicyclopentacyclopenta[cd]-s-indacene (PIN)

A less preferred hydrocarbon parent component is selected that will permit a fusion name. Second and third choice parent components are chosen according to the seniority order for selecting the senior parent component (see P-25.3.2.4).

Examples:

cyclobuta[1,7]indeno[5,6-b]naphthalene (PIN)
(anthracene cannot be selected as senior parent component;
naphthalene, not indene, is next in seniority order for selection as a parent component)

10-azacyclobuta[1,7]indeno[5,6-b]anthracene (PIN)
[neither quinoline nor pyridine can be used as the senior parent component
because neither naphthalene nor anthracene, respectively, can be used as the senior attached component;
therefore ‘a’ nomenclature must be used (see P-25.5.1) and
since the preferred hydrocarbon tetracene cannot be used,
the next senior component, anthracene, is chosen as the parent component]

A bridged fused system (see Section P-25.4) is used to generate names for structures that cannot be named by normal fusion nomenclature. A fused system is first envisaged; additional rings are created by using bridges.

Examples:

12,19:13,18-di(metheno)dinaphtho[2,3-a:2′,3′-o]pentaphene (PIN)

1-oxa-5,9,2-(epiethane[1,1,2]triyl)cycloocta[cd]pentalene (PIN)

The λ-convention is used to describe atoms with nonstandard bonding numbers (ref. 13) in fused ring systems. The symbol λⁿ is used, where ‘n’ is the bonding number of the atoms; it is cited immediately after the locant denoting the atom having the nonstandard bonding number. The symbol H, denoting indicated hydrogen atom(s) with the appropriate locant(s), if present, is cited at the front of the complete name.

The λ symbol is used with all ring systems described in this section with retained and fusion names, bi- and polyalicyclic as well as heterocycles formed by skeletal replacement (‘a’) nomenclature, as described above in P-25.5.1. Atoms with nonstandard bonding numbers in fused ring systems are indicated only in the complete ring system, not in component rings. Names and numbering are unchanged, unless a choice must be made between two otherwise equivalent atoms; in which case, low locants are attributed to atoms with the higher bonding numbers, for example λ⁶ before λ⁴.

Examples:

7λ⁴-[1,2]dithiolo[5,1-e][1,2]dithiole (PIN)

2H-5λ⁵-phosphinino[3,2-b]pyran (PIN)

1λ⁴,5-benzodithiepine (PIN)

Examples:

2H-5λ⁴-dibenzo[b,d]thiophene (PIN)

3H-3λ³,2,4-benziodadioxepine (PIN)

3H-2λ⁴-cyclohepta[c]thiopyran (PIN)

The treatment of double bonds is formalized in fusion nomenclature. A maximum of noncumulative double bonds must be present in fused and bridged fused systems. When hydrogen atoms are in excess at specific positions, they are denoted in names by indicated hydrogen atom(s). However, the presence of cumulative formal double bonds is also possible. This feature is expressed by the δ-convention (delta-Convention) (see ref. 24). These two aspects of fused and bridged fused systems are described in this Section.

P-25.7.1 Indicated hydrogen

P-25.7.1.1 Maximum number of noncumulative double bonds

The names of polycyclic fused ring systems are considered to correspond to the structure with the maximum number of noncumulative double bonds consistent with the appropriate bonding number of the skeletal atoms. To achieve this result, components are fused together and noncumulative double bonds are introduced into the completed fused system. Hydrogen atoms not attached to atoms connected by double bonds are denoted as indicated hydrogen atom(s).

Examples:

pyrrolo[3,2-b]pyrrole (PIN)

2H-1,3-benzoxathiole (PIN)

Example:

3λ⁴-pyrido[3,2-d][1,3]thiazine (PIN)

Examples:

1,4-epoxy-4a,8a-ethanonaphthalene (PIN)

2H,6H-2,5-(epiethanylylidene)[1,3]dioxolo[4,5-b]oxepine (PIN)

2H-2λ⁵-2,6-(epiethanylylidene)isophosphinoline (PIN)

9H,13H-9,10-[3,4]epipyrroloacridine (PIN)

If it is necessary to identify isomers that differ only by virtue of the location of localized double bonds, this differentiation is indicated by the use of the Greek letter Δ. The locant cited corresponds to the first cited locant of the localized double bond.

Examples:

1,6-dimethyl-Δ¹-heptalene (PIN)

1,6-dimethyl-Δ^1(10a)-heptalene (PIN)

P-25.7.1.3.1 When a name applies equally to two or more isomeric systems with the maximum number of noncumulative double bonds and when the name can be made specific by indicating the position of one or more hydrogen atoms in the structure, this specifically is accomplished by adding to the name the italicized symbol H for each of these hydrogen atoms, preceded by the appropriate locant(s).

In preferred IUPAC names, all indicated hydrogen atoms must be cited when the names are constructed in accordance with the principles of fusion nomenclature

In general IUPAC nomenclature, omission of indicated hydrogen atoms is permitted in some parent fused ring systems, when unsubstituted, for example indene rather than 1H-indene, but 1H-indene-3-carboxylic acid. Omission of indicated hydrogen in general nomenclature is permitted in the following ring systems:

Omission of indicated hydrogen is also permitted in general nomenclature if no ambiguity would result, for example 1,3-benzodioxole, rather than 2H-1,3-benzodioxole.

P-25.7.1.3.2 Indicated hydrogen with ortho-, and ortho and peri-fused systems

Indicated hydrogen is identified by the locant of the relevant position and cited at the front of the names of the whole ring system, including replacement terms, if any.

Examples:

1H-pyrrolo[3,2-b]pyridine (PIN)

6H-1,7-dioxacyclopenta[cd]indene (PIN)

1H,3H-thieno[3,4-c]thiophene (PIN)

All indicated hydrogen atoms are indicated at the front of the complete name.

Note: Citation of indicated hydrogen in bridged fused ring systems in this manner is completely consistent with its citation for spiro atoms (see P-24.3.2) and ring assemblies (see P-28.2.3).

Examples:

2H,7H-4a,7-ethano-1-benzopyran (PIN)
2H,7H-4a,7-ethanochromene

1H-3a,7-ethanoazulene (PIN)

1H,15H-12,5-[2,3]epipyranoanthra[2,3-f]isoindole (PIN)

The presence of contiguous formal double bonds at a skeletal atom in a cyclic parent hydride whose name normally implies the maximum number of noncumulative double bonds is described by the symbol δ^c, where ‘c’ is the number of double bonds directly attached to the identified atom (see ref. 24). The δ^c symbol is cited immediately after an expressed locant for the skeletal atom in the name of the compound and follows the λⁿ symbol, if present.

Examples:

8δ²-benzo[9]annulene (PIN)

2λ⁴δ²,5λ⁴δ²-thieno[3,4-c]thiophene (PIN)

2λ⁴δ²-thieno[3,4-c]thiophene (PIN)

2λ⁵δ²-6,2-(epiethanylylidene)isophosphinoline (PIN)

In this subsection rings and ring systems are listed in decreasing order of seniority for selection as a parent component. The first list contains heterocyclic parent components, the second hydrocarbon parent components.

P-25.8.1 Partial list of heterocyclic parent components in decreasing order of seniority

The following list contains parent heterocycles arranged in decreasing order of seniority for selection as the parent component for fusion names. Ring systems containing Hg as given in ref. 4 are not included in these recommendations.

The parent heterocycles are arranged by ring analysis, i.e., in decreasing order of number rings, ring size, and in accordance with the priority given to heteroatoms, N, O, S, Se.

10H-phenoxazine

Note 1: Indicated hydrogen atoms are not shown in this kind of listing.

Note 2: The seniority order given below is used to select parent components in fusion nomenclature.

Note 3: In the order of seniority used by CAS quinolizine precedes quinoline and isoquinoline; and indolizine preceeds indole and isoindole..