FR-8.3 Naming of Bridges

Contents of Section

FR-8.3 Naming of bridges
FR-8.3.1 Simple bivalent bridges
FR-8.3.2 Simple polyvalent bridges
FR-8.3.3 Composite bridges
FR-8.3.4 Elision
References for this Section

Continue with FR-8.4 Naming of bridged fused ring systems

FR-8.3.1 Simple bivalent bridges

a. An acyclic bivalent hydrocarbon bridge is named as a prefix derived from the corresponding hydrocarbon name by changing the final 'e' to 'o'. The locant of a double bond, if present, is indicated in square brackets between the hydrocarbon prefix and the ending '-eno'. If there is a choice low numbers are preferred (e.g. prop[1]eno rather than prop[2]eno). Note that this is not the locant used in the final numbering of the bridged fused ring system (see FR-8.5).

Examples:

methano	-CH2-
ethano	-CH2CH2-
propano	-CH2CH2CH2-
butano	-CH2CH2CH2CH2-
etheno	-CH=CH-
prop[1]eno	-CH=CHCH2-	CAS and Beilstein use propeno
but[1]eno	-CH=CHCH2CH2-	Formerly [1]buteno which is still used by CAS; Beilstein uses but-1-eno
but[2]eno	-CH2CH=CHCH2-	Formerly [2]buteno which is still used by CAS; Beilstein uses but-2-eno
buta[1,3]dieno	-CH=CHCH=CH-	Formerly [1,3]butadieno which is still used by CAS; Beilstein uses buta-1,3-dieno

b. A monocyclic hydrocarbon bridge other than benzene is named by the same prefix as that used as a fusion prefix (FR-2.1.1). To distinguish between the two the prefix for a bridge has 'epi-' added in front of the name. The bridge is assumed to have the maximum number of non-cumulative double bonds consistent with the attachments to the fused ring system or to other bridges. The positions of the free valencies are indicated by the relevant locants in square brackets directly in front of the bridge name. Note that these locants are not those assigned to the final structure (see FR-8.6).

Examples:

epicyclopropa		Indicated hydrogen is needed to locate double bonds (see FR-9.3.2). Note [1,2] is implied but not stated. CAS uses endo-cyclopropa, Beilstein uses [1,2]cycloprop-2-eno
[1,2]epicyclopenta		Indicated hydrogen is needed to locate double bond (see FR-9.3.2). CAS uses [1',2']-endo-cyclopenta, Beilstein uses [1,2]cyclopenta[2,4]dieno
[1,5]epicycloocta		CAS uses [1',5']-endo-cycloocta, Beilstein uses [1,5]cycloocta-1,3,5,7-tetraeno

c. Any other cyclic hydrocarbon is named as a prefix derived from the corresponding unsaturated hydrocarbon name in Appendix 1 by replacing the terminal '-e' by '-o'. If the name of the bridge is the same as the fusion prefix the bridge is indicated by adding 'epi-' in front of the name. The 'i' is elided if followed by a vowel. Locants are used in the same way as in section b.

Examples:

[1,2]benzeno		Formerly o-benzeno which is used by Beilstein.CAS uses [1',2']-benzeno
[1,3]benzeno		Formerly m-benzeno which is used by Beilstein; CAS uses [1',3']-benzeno
[2,3]naphthaleno		CAS uses [2',3']-naphthaleno
[1,3]epindeno		Indicated hydrogen is needed to locate the double bonds (see FR-9.3.2). CAS uses [1',3']-endo-indeno, Beilstein uses [1,3]-epiindeno

d. An acyclic heteroatom bridge is named by the appropriate prefix. Non-standard valence states are given using the -convention (ref 17).

Examples:

epoxy	-O-	Beilstein uses epioxido
epidioxy	-OO-	Beilstein uses epidioxido
epitrioxy	-OOO-	Beilstein uses epitrioxido
epithio	-S-	Beilstein uses episulfano
4-sulfano	-SH2-	Beilstein uses epi-4-sulfano
6-sulfano	-SH4-
epidithio	-SS-	Beilstein uses epidisulfano
episeleno	-Se-	Beilstein uses episelano
epitelluro	-Te-	Beilstein uses epitellano
epimino	-NH-	formerly imino which is still used by CAS; Beilstein uses epiazano
diazano	-NHNH-	formerly biimino which is still used by CAS; Beilstein uses epidiazano
diazeno	-N=N-	formerly azo which is still used by CAS; Beilstein uses epidiazeno
triaz[1]eno	-N=NNH-	formerly azimino which is still used by CAS; Beilstein uses epitriazeno or for -N=N-NR- epitriaz-1-eno
phosphano	-PH-	CAS uses phosphinidene; Beilstein uses epiphosphano
arsano	-AsH-	CAS uses arsinidene; Beilstein uses epiarsano
stibano	-SbH-	CAS uses stibylene; Beilstein uses epistibano
silano	-SiH2-	Beilstein uses episilano
germano	-GeH2-	Beilstein uses epigermano
stannano	-SnH2-
borano	-BH-	CAS uses borylene; Beilstein uses epiborano

Note

CAS names hydrocarbons and non-nitrogenous heterocyclic systems with epimino bridges by use of the suffix -imine preceded by the appropriate locants and with elision of a final 'e' if present e.g. naphthalen-1,4-imine.

e. A heterocyclic bridge is named as a prefix derived from the corresponding heterocyclic compound listed in Appendix 2 by adding an 'o' with elision of a final 'e' if present. If the heterocyclic system requires the citation of locants these are given in square brackets in front of the name. If the name of the bridge is the same as the fusion prefix the bridge is indicated by adding 'epi-' in front of the name. The 'i' is elided if followed by a vowel.

Examples:

epoxireno		CAS uses endo-oxireno, and endo-oxirano for dihydroepoxireno; Beilstein uses etheno and ethano with epoxy substituent prefixes.
[2,3]furano		CAS uses [2',3']-furano
[3,4]furano		CAS uses [3',4']-furano
[2,5]furano		CAS uses [2',5']-furano
[2,3]epipyrano		Indicated hydrogen is needed to locate the double bond see (FR-9.3.2). CAS uses [2',3']-endo-pyrano
[3,4]epi[1,2,4]triazolo		CAS uses [3',4']-endo-[1,2,4]triazolo

FR-8.3.2 Simple polyvalent bridges

Polyvalent bridges are cited within round brackets.

Note Brackets are not used by CAS or Beilstein.

a. A polyvalent bridge consisting of one atom (other than hydrogen) is named by the appropriate prefix.

Examples:

(metheno)	-CH=	CAS and Beilstein use metheno without brackets
(methanetriyl)		CAS uses metheno; Beilstein uses epimethanetriyl
(methanediylylidene)		CAS uses methyno; Beilstein uses epimethanediylylidene
(methanetetrayl)		CAS uses methyno; Beilstein uses epimethanetetrayl
(azeno)	-N=	CAS uses nitrilo; Beilstein uses epiazanylylidene
(azanetriyl)		CAS uses nitrilo; Beilstein uses epiazanetriyl
(phospheno)	-P=	CAS uses phosphinidyne; Beilstein uses epiphosphanylylidene
(phosphanetriyl)		CAS uses phosphinidyne; Beilstein uses epiphosphanetriyl

b. A polyvalent polyatomic bridge is named as the appropriate polyvalent group (see A-4.4, 4.5 in ref 3 and R-2.5, R-3.1.4 in ref 3a). If necessary the positions of the free valencies are indicated by the appropriate locants directly in front of the associated ending. The ending '-ylidene' is restricted to those cases where there is a double bond between the bridge and the fused ring system. If there is a choice of numbering of the bridge preference is given to low numbers for (a) -yl, (b) -ylidene, (c) double bonds.

Note

The previous edition of these rules (ref 3) also used '-ylidene' to indicate bonds to two different positions (and '-ylidyne' to indicate bonds to three positions) and cited the locant(s) nearest to the hydrocarbon or numerical prefix in front of that prefix. This practice is still used by CAS.

Examples:

(ethanylylidene)	-CH2CH=	Beilstein uses epiethanylylidene
(ethane[1,1,2]triyl)		CAS uses ethanylylidene; Beilstein uses epiethane-1,1,2-triyl
(ethanediylidene)	=CHCH=	Beilstein uses epiethanediylidene
(ethane[1,1]diyl[2]ylidene)		Beilstein uses epiethane-1,1-diyl-2-ylidene
(ethane[1,1,1,2]tetrayl)		CAS uses ethanylylidyne
(ethane[1,1,2,2]tetrayl)		CAS uses ethanediylidene; Beilstein uses epiethane-1,1,2,2-tetrayl
(ethene[1,1,2]triyl)		CAS uses ethenylylidene
(propane[1,2,3]triyl)
(propan[1]yl[3]ylidene)	-CH2CH2CH=
(propane[1,1,3]triyl)		CAS uses [1]propanyl[3]ylidene
(propane[1,2]diyl[3]ylidene)
(propane[1,1,2,3]tetrayl)		CAS uses [1,2]propanediyl[3]ylidene
(propane[1,1,1,3]tetrayl)		CAS uses [1]propanyl[3]ylidyne

(prop[1]en[1]yl[3]ylidene)	-CH=CHCH=
(prop[2]ene[1,1,3]triyl)		CAS uses [1]propen[1]yl[3]ylidene
(prop[2]ene[1,3]diyl[1]ylidene)		CAS uses [1]propen[1]yl[3]ylidyne
(propan[1]yl[2,3]diylidene)
(propan[2]yl[1,3]diylidene)
(propane[1,1,3]triyl[2]ylidene)
(propane[1,2,2]triyl[3]ylidene)		CAS uses [1]propanyl[2,3]diylidene
(propane[1,1,2,2,3]pentayl)
(buta[1,3]diene[1,1,4]triyl)		CAS uses [1,3]butadien[1]yl[4]ylidene
(but[1]ene[1,3]diyl[4]ylidene)
(but[3]ene[1,1,2,4]tetrayl)
(diazanediylidene)	=N-N=	formerly azino which is still used by CAS; Beilstein uses epidiazanediylidene
(pyrrol[2]yl[5]ylidene)		CAS uses [2]pyrrolyl[5]ylidene; Beilstein uses epipyrrol-2-yl-5-ylidene

FR-8.3.3 Composite bridges

Composite bridges are named by combining the names of two or more simple bridge prefixes. However see FR-8.5 for a different treatment when the fused ring system is named using replacement nomenclature. Unless cited first ep(i) is omitted. The prefixes are cited without elision in order starting from the preferred terminal prefix based on the following criteria applied in turn until a decision is reached:

a. The simple bridge containing the preferred heteroatom [see table 1 (ref 9)].

b. The simple bridge containing the preferred ring system (see FR-2.3).

c. The simple bridge containing the longer acyclic chain.

d. The simple bridge which occurs first alphabetically.

Composite bridges are cited within round brackets. Locants associated with a cyclic bridge component are cited in attachment order. If an acyclic bridge component requires internal numbering it is numbered in the direction implied by the bridge name (cf. numbering in FR-8.3.1.a). Acyclic bridge components in a composite bridge are only attached to other components through terminal positions e.g. only 1 and 3 of propane-1,2,3-triyl. If polyvalent acyclic bridge components are used care must be taken to check that there is no ambiguity. If there is ambiguity then alternative arrangements such as a dependent bridge must be considered.

Examples:

(epoxymethano)	-OCH2-	Beilstein uses oxaethano
(epiminoethano)	-NHCH2CH2-	formerly iminoethano which is still used by CAS; Beilstein uses [1]azapropano
(epoxyethane[1,1,2]triyl)		CAS uses (epoxy[2]ethanyl[1]ylidene); Beilstein uses epi[1]oxapropane-1,2,3-triyl
(epoxyethane[1,2,2]triyl)		CAS uses (epoxyethanylylidene); Beilstein uses epi[1]oxapropane-1,3,3-triyl
(epoxyprop[1]eno)	-OCH=CHCH2-
(epoxyprop[2]eno)	-OCH2CH=CH-	CAS uses (epoxy[2]propeno); Beilstein uses [1]oxabut-3-eno
([1,4]benzenomethano)		Beilstein prefers cyclophane name here
(epoxy[1,4]benzeno)		Beilstein prefers cyclophane name here
(ethanoiminomethano)	-CH2CH2NHCH2-	CAS uses (ethaniminomethano); Beilstein uses [2]azabutano
(methanooxymetheno)	-CH2OCH=	CAS uses (methanoxymetheno); Beilstein uses epi[2]oxapropan-1-yl-3-ylidene
(methanetriyloxymethano)		CAS uses (methanoxymetheno) for this also; Beilstein uses epi[2]oxapropane-1,1,3-triyl
(epoxymethanoazenometheno)	-OCH2N=CH-	CAS uses (epoxymethanonitrilometheno); Beilstein uses [1]oxa[3]azabut-3-eno
(epoxymethenoazenomethano)	-O-CH=N-CH2-	CAS uses (epoxymethenonitrilomethano); Beilstein uses [1]oxa[3]azabut-2-eno
([3,4]furanomethano)		Beilstein prefers a cyclophane name
([2,3]furanomethano)		Beilstein prefers a cyclophane name
([3,2]furanomethano)		Beilstein prefers a cyclophane name
(ethano[2,5]pyrrolomethano)		Beilstein prefers a cyclophane name

FR-8.3.4 Elision

Vowels are not elided for bridges.

CAS elides the terminal 'a' of a monocyclic hydrocarbon bridge when followed by a vowel. With composite bridges the terminal 'o' of benzeno or an acyclic component is elided when followed by a component of the composite bridge which starts with a vowel (iminoethano is an exception). In all other cases a terminal 'a', 'e', or 'o' is not elided. Examples of CAS index names are:

3. IUPAC Nomenclature of Organic Chemistry, Sections A and B, 1st edition, 1958; 2nd edition, 1966; 3rd edition (combined with section C), 1971; 4th edition (combined with sections C, D, E, F and H), 1979.

3a. A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993, Blackwell Scientific Publications, 1993.

9. IUPAC, Revision of the extended Hantzsch-Widman system of nomenclature for heteromonocycles, Pure Appl. Chem. 55, 409-416 (1983), see also ref 3a, pp. 40-44.

17. IUPAC, Treatment of variable valence in organic nomenclature (lambda convention), Pure Appl. Chem. 56, 769-778 (1984).

Return to home page for Fused Ring Nomenclature.