Ring Systems

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Polycyclic aromatic hydrocarbons consisting of fused benzene rings in a rectilinear arrangement. NOC Rule A-21.2, GNOC Recom. R-

alicyclic compounds:

Aliphatic compounds having a carbocyclic ring structure which may be saturated or unsaturated, but may not be a benzenoid or other aromatic system.


Mancude monocyclic hydrocarbons without side chains of the general formula CnHn (n is an even number) or CnHn+1 (n is an odd number). In systematic nomenclature an annulene with seven or more carbon atoms may be named [n]annulene, where n is the number of carbon atoms. GNOC Recom. R- E.g. [9]annulene for cyclonona-1,3,5,7-tetraene.


Carbenes, derived by formal insertion of a divalent carbon atom into an even-membered annulene. E.g. cycloheptatrienylidene

ansa compounds:

Benzene derivatives having para positions (or meta) bridged by a chain (commonly 10 to 12 atoms long) (Latin ansa, handle). By extension, any arene bridged by a chain constrained to lie over one of the two faces of the arene. Cf. cyclophanes.

antiaromatic compounds:

Compounds that contain 4n (n not equal to 0) pi-electrons in a cyclic planar, or nearly planar, system of alternating single and double bonds. Cf. aromatic compounds. E.g. cyclobuta-1,3-diene.


Monocyclic and polycyclic aromatic hydrocarbons. NOC Rule A-12.4. See aromatic compounds.

arene epoxides:

Epoxides derived from arenes by 1,2-addition of an oxygen atom to a formal double bond. E.g. 5,6-epoxycyclohexa-1,3-diene.

(Common usage has extended the term to include examples with the epoxy group bridging nonadjacent atoms.)

arene oxides: See arene epoxides.

arenium ions:

Cations derived formally by the addition of a hydron or other cationic species to any position of an arene. E.g. C6H7+, benzenium. Cf. aryl cations.

The term includes:

1. Arenium sigma-adducts (Wheland intermediates) such as (which are considered to be intermediates in electrophilic aromatic substitution reactions) and other cyclohexadienyl cations.

2. Arenium pi-adducts, such as

GTPOC [sigma]-adduct (sigma adduct), [pi]-adduct (pi adduct).

arenonium ions: An obsolescent name for arenium ions.

aromatic compounds:

Historically, the term "aromatic" originally referred to the smell of selected compounds that later were found to contain benzene or fused benzene rings in the structure. In a structural sense, it designates compounds that, in accordance with the theory of Hückel, have a cyclic, delocalized (4n+2) pi-electron system. This includes arenes and their substitution products. E.g. benzene, naphthalene, toluene. The term has been generalized to include aromatic heterocyclic structures, such as thiophene and pyridine, but it is more precise to call compounds of the latter type heteroaromatic. See also arenes, heteroarenes. GTPOC (aromatic).

aryl cations:*

Carbocations formally derived by removal of a hydride ion from a ring carbon atom of an arene. Cf. arenium ions. E.g. phenylcation or phenylium

aryl groups:*

Groups derived from arenes by removal of a hydrogen atom from a ring carbon atom. NOC Rule A-13.5. Groups similarly derived from heteroarenes are sometimes subsumed in this definition (see heteroaryl groups). E.g. o-tolyl.

arylene groups:*

Bivalent groups derived from arenes by removal of a hydrogen atom from two ring carbon atoms. A synonym is arenediyl groups. NOC Rule A-13.5. E.g. o-phenylene or benzene-1,2-diyl


Hydrocarbons derived formally from arenes by the abstraction of two hydrogen atoms from adjacent carbon atoms; thus 1,2-didehydroarenes. Arynes are commonly represented with a formal triple bond. Cf. benzynes, dehydroarenes, heteroarynes. E.g. benzyne

benzenium ions:*

Arenium ions, derived from benzene or substituted derivatives. RNRI Rule RC-

benzenonium ions: Obsolescent name for benzenium ions.

benzylic groups:

Arylmethyl groups and derivatives formed by substitution: ArCR2- . Benzyl, C6H5CH2- , is the prototype. The term 'benzylic position' or 'benzylic site' refers to the saturated carbon atom. A group, such as -OH, attached at a benzylic site is sometimes referred to as 'benzylic'.

benzylic intermediates:

Carbanions, carbenium ions, or radicals, derived formally by detachment of one hydron, hydride or hydrogen, respectively, from the CH3 group of toluene or substitution derivatives thereof. E.g. C6H5CH2. benzyl radical.


1,2-Didehydrobenzene (the aryne derived from benzene) and its derivatives formed by substitution. Cf. arynes, dehydroarenes . The terms m- and p-benzyne are occasionally, but erroneously, used for 1,3- and 1,4-didehydrobenzene, respectively (benzene-1,3-diyl and benzene-1,4-diyl, according to RNRI Rule RC-


Bicyclic alkenes having a double bond between the bridgehead atoms and a trans attachment of each branch to the double bond. Thus trans-bicyclo[m.n.0]alk-1(m+2)-enes. J. Marshall, Acc. Chem. Res., 13, 213-218 (1980).


Originally macrocyclic compounds capable of assuming a basket (or "calix") shaped conformation. They are formed from p-hydrocarbyl phenols and formaldehyde. The term now applies to a variety of derivatives by substitution of the hydrocarbon cyclo{oligo[(1,3-phenylene) methylene]}. C. D. Gutsche, Calixarenes, The Royal Society of Chemistry, 1989.

carbocyclic compounds:

Cyclic compounds in which all of the ring members are carbon atoms. E.g. 1,2-dihydronaphthalene. See also homocyclic compounds.

catenanes/catena compounds:

Hydrocarbons having two or more rings connected in the manner of links of a chain, without a covalent bond. More generally, the class catena compounds embraces functional derivatives and hetero analogues. E.g. a [2]catenane


Compounds constrained by structure to having or accommodating a cavity large enough to host other molecules. See inclusion compounds.

coronands/coronates: See crown compounds.


2H-Chromen-2-one (older name 1,2-benzopyrone), trivially named coumarin, and its derivatives formed by substitution. Cf. isocoumarins.

crown compounds:

Macrocyclic polydentate compounds, usually uncharged, in which three or more coordinating ring atoms (usually oxygen or nitrogen) are or may become suitably close for easy formation of chelate complexes with metal ions or other cationic species. (Planar analogues, such as porphyrins, are excluded.) They are also known as coronands and the chelate complexes are called coronates. GTPOC (crown).

E.g. a crown ether (a subclass containing only oxygen as coordinating atom.)

M. Hiraoka, Crown Compounds: their Characteristics and Applications, Elsevier Science Publishers, 1982.


Cryptands are macrobicyclic, macrotricyclic, etc. compounds generally having nitrogen atoms at the bridgehead positions, having sufficient space within its cage structure for polydentate ligation to metal ions or other cationic species; the resulting complexes are called cryptates. E. Weber and F. Vögtle, Inorg. Chim. Acta, 45, L65-L67 (1980).


Saturated monocyclic hydrocarbons (with or without side chains). See alicyclic compounds. NOC Rule A-11.1. E.g. cyclobutane. Unsaturated monocyclic hydrocarbons having one endocyclic double or one triple bond are called cycloalkenes and cycloalkynes, respectively. Those having more than one such multiple bond are cycloalkadienes, cycloalkatrienes, etc. The inclusive terms for any cyclic hydrocarbons having any number of such multiple bonds are cyclic olefins or cyclic acetylenes.

cycloalkyl groups:*

Univalent groups derived from cycloalkanes by removal of a hydrogen atom from a ring carbon atom. E.g. 2-methylcyclopropyl. NOC Rules A-11.1, A-11.2.

cyclohexadienyl cations:

A subclass of arenium ions.


The term originally applied to compounds having two p-phenylene groups held face to face by -[CH2]n- bridges. It now designates compounds having (i) mancude-ring systems, or assemblies of mancude-ring systems, and (ii) atoms and/or saturated or unsaturated chains as alternate components of a large ring. GNOC Recom. R-2.4.5. Many chemists include in this class any bridged aromatic system, irrespective of the attachment positions of the bridge. E.g. [2.2](1,4)(1,4)cyclophane or 1(1,4),4(1,4)-dibenzenacyclohexaphane according to the provisional CNOC document on Phane Nomenclature, Part 1.

F.N. Diederich, Cyclophanes, The Royal Society of Chemistry, 1991.


Species, usually transient, derived formally by the abstraction of a hydrogen atom from each of two ring atoms of an arene. The name for specific compounds requires the numerical prefix di-.

E.g. 1,8-didehydronaphthalene or naphthalene-1,8-diyl. RNRI Rule RC-81.3.1.

1,2-Didehydroarenes are called arynes and are commonly represented with a formal triple bond.

dehydrobenzenes: A subclass of dehydroarenes. See benzynes.


Compounds of the general formula shown below. They may be considered to be spiro compounds having bridges of carbon atoms connecting the [alpha] and [alpha]' positions. A. Greenberg and J. Liebman, Strained Organic Molecules, Academic Press, 1978, p. 373; R. Keese, in Organic Synthesis: Modern Trends, O. Chizhov, Ed., Blackwell Scientific Publications, 1988, p. 43 ff.


Compounds composed solely of an even number of carbon atoms, which form a cage-like fused-ring polycyclic system with twelve five-membered rings and the rest six-membered rings. The archetypal example is [60]fullerene, where the atoms and bonds delineate a truncated icosahedron. The term has been broadened to include any closed cage structure consisting entirely of three-coordinate carbon atoms. Buckminsterfullerenes, W. E. Billups and M. A. Ciufolini, Eds., VCH, 1993.


The hydrocarbon fulvalene and its derivatives formed by substitution (and by extension, analogues formed by replacement of one or more carbon atoms of the fulvalene skeleton by a heteroatom).



The hydrocarbon fulvene and its derivatives formed by substitution (and by extension, analogues formed by replacement of one or more carbon atoms of the fulvene skeleton by a heteroatom).



ortho-Fused polycyclic aromatic or heteroaromatic compounds in which all rings (minimum five) are angularly arranged so as to give helically shaped molecules, which are thus chiral. GNOC Recom. R- E.g. hexahelicene

hetarenes: Synonymous with heteroarenes.

hetaryl groups: Synonymous with heteroaryl groups.

hetarynes: Synonymous with heteroarynes.


Heterocyclic compounds formally derived from arenes by replacement of one or more methine (-C=) and/or vinylene (-CH=CH-) groups by trivalent or divalent heteroatoms, respectively, in such a way as to maintain the continuous [pi]-electron system characteristic of aromatic systems and a number of out-of-plane [pi]-electrons corresponding to the Hückel rule (4n + 2); an alternative term is hetarenes. Comprehensive Heterocyclic Chemistry, Vol. 1, Ed. O. Meth-Cohn, Pergamon, 1984, p. 3. E.g.

heteroaryl groups:

The class of heterocyclyl groups derived from heteroarenes by removal of a hydrogen atom from any ring atom; an alternative term is hetaryl. E.g. 2-pyridyl (pyridin-2-yl), indol-1-yl.


Compounds derived from heteroarenes by replacement of a formal carbon-carbon double bond by a formal triple bond (with loss of two hydrogen atoms). Also known as hetarynes and as 1,2-didehydroheteroarenes. See arynes. H. J. den Hertog and H. C. van der Plas, in Advances in Heterocyclic Chemistry, Ed. A. R. Katritzky, Vol. 4, Academic Press, 1965, p. 121. E.g.

heterocyclic compounds:

Cyclic compounds having as ring members atoms of at least two different elements. E.g. quinoline, 1,2-thiazole, bicyclo[3.3.1]tetrasiloxane. Cf. homocyclic compounds, carbocyclic compounds.

heterocyclyl groups:

Univalent groups formed by removing a hydrogen atom from any ring atom of a heterocyclic compound. E.g. 4-pyridyl, pyrrolidin-1-yl, pyrrolidin-2-yl. See organyl.

homocyclic compounds:

Cyclic compounds having as ring members atoms of the same element only. E.g. benzene, pentazole, cyclohexasilane.


Isocoumarin (1H-isochromen-1-one) and its derivatives formed by substitution. Cf. coumarins.

isocyclic compounds: A less preferred synonym for homocyclic compounds.

mancude-ring systems:

Rings having (formally) the maximum number of noncumulative double bonds are termed mancude. E.g. benzene, indene, indole, 4H-1,3-dioxine. Also called mancunide-ring systems.

mancunide-ring systems: see mancude-ring systems.


1,2,3-Trioxolanes, the primary products of the reaction of ozone at a carbon-carbon double bond. Cf. ozonides.


Cycloalkanes especially cyclopentane, cyclohexane and their alkyl derivatives. The term seems to be obsolescent, except in the petrochemical industry.


The 1,2,4-trioxolanes formed by the reaction of ozone at a carbon-carbon double bond, or the analogous compounds derived from acetylenic compounds. Cf. molozonides.


Tricyclic saturated hydrocarbons having two bridgehead carbon atoms joined through four bridges, systematically named tricyclo[m.n.o.p1,(m+2)]alkanes, have been referred to as [m.n.o.p]paddlanes (when p = 0, the compounds are propellanes).


Polycyclic hydrocarbons of the (CH)n series having skeletons corresponding to the regular and semiregular geometrical solids. E.g. cubane.


Saturated or unsaturated, respectively, polycyclic hydrocarbons consisting of fused five-membered rings, commonly but not necessarily incorporating the skeleton of quinacene (tricyclo[,10]deca-2,5,8- triene): E.g. bicyclo[3.3.0]octane, dodecahedrane.


Tricyclic saturated hydrocarbons, systematically named tricyclo[a.b.c.01,(a+2)]alkanes; have been referred to as [a.b.c]propellanes. Cf. paddlanes.

[4.1.1]propellane [4.4.4]propellane


Mancude assemblies of three carbocyclic rings, a six-membered quinonoid ring bonded at the 1,4-positions to odd-membered rings which differ in ring size by two. E.g.

[3.6.5]quinarene, [5.6.7]quinarene


Molecules in which a ring encloses another, rod-like molecule having end groups too large to pass through the ring opening, and thus holds the rod-like molecule in position without covalent bonding. Cf. catenanes.

G = end groups

spiro compounds:

Compounds having one atom (usually a quaternary carbon) as the only common member of two rings. NOC Rule A-41. E.g. spiro[cyclopentane-1,2'-indene], spiro[4.4]nonane

Wheland intermediates: See under arenium ions.

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