FR-9 Indicated Hydrogen

Contents of Section

FR-9. Indicated Hydrogen
FR-9.1 Maximum number of non-cumulative double bonds
FR-9.2 Localised double bonds
FR-9.3 Citation of indicated hydrogen
FR-9.3.1 Ortho- or ortho- and peri-fused ring systems
FR-9.3.2 Bridged systems
FR-9.4 Partially hydrogenated fused ring stems
References for this Section

Continue with References

FR-9.1 Maximum number of non-cumulative double bonds

The names of polycyclic fused ring systems are considered to correspond to the compound with the maximum number of non-cumulative double bonds consistent with the appropriate valency of the skeletal atoms.

Examples:

pyrrolo[3,2-b]pyrrole

2H-1,3-benzoxathiole
2H may be omitted

If non-standard valence states are present this must be indicated using the λ-convention (ref 17) [and if necessary the δ-convention (ref 19)]. The corresponding valency is used in assigning the non-cumulative double bonds.

Examples:

3λ⁴-pyrido[3,2-d][1,3]thiazine

2λ⁴δ²-thieno[3,4-c]thiophene

With bridged systems the assignment of non-cumulative double bonds to the parent fused ring system is applied after allowance for the bonds existing between the bridge(s) and the fused ring system. Rings which form part of a bridge are treated separately, after consideration of free valencies implied by the name.

Examples:

1,4-epoxy-4a,8a-ethanonaphthalene

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

2λ⁵2δ²-6,2-(ethanylylidene)isophosphinoline

2H-2λ⁵-2,6-(ethanylylidene)isophosphinoline

9H,13H-9,10-[3,4]epipyrroloacridine

FR-9.2 Localised double bonds

If it is necessary to identify isomers which differ only by virtue of the location of localised double bonds this is indicated by the use of the Greek letter Δ. The locant(s) cited correspond to the lowest numbered atom of a localised double bond.

Examples:

1,6-dimethyl-Δ¹-heptalene

1,6-dimethyl-Δ^1(10a)-heptalene

FR-9.3 Citation of indicated hydrogen

When a name applies equally to two or more isomeric systems with the maximum number of non-cumulative double bonds, and when the name can be made specific by indicating the position of one or more hydrogen atoms in the structure, this is accomplished by modifying the name with a locant, followed by an italic H for each of these hydrogen atoms. These atoms are called indicated hydrogen.

Notes

1. Indicated hydrogen is often omitted for the normally encountered isomer of some ring systems (see rule B-2.11 ref 3, and note a to Table 20 and 23 ref 3a) i.e. carbazole, fluorene, imidazole, indazole, indene, indole, isochromene, isoindole, perimidine, phenalene, phenothiazine, phenoxazine, purine, pyrazole, pyrrole, selenoxanthene, telluroxanthene, thioxanthene, xanthene.

2. Indicated hydrogen may be omitted where there is no ambiguity. For example at a position located between two divalent atoms (see example 2 under FR-9.1).

FR-9.3.1 Ortho- or ortho- and peri-fused ring systems

If indicated hydrogen is needed it is identified by the locant of the relevant position (see FR-5) cited in front of the name of the whole ring system, including replacement terms, if used.

Examples:

1H-pyrrolo[3,2-b]pyridine

6H-1,7-dioxacyclopenta[cd]indene

1H,3H-thieno[3,4-c]thiophene

FR-9.3.2 Bridged systems

If indicated hydrogen is necessary to distinguish between isomers of the fused ring system and/or of the bridge indicated hydrogen is cited in front of the name.

Examples:

2H,7H-4a,7-ethanochromene
CAS name 7H-4a,7-ethano-2H-1-benzopyran

1H-3a,7-ethanoazulene

1H,15H-12,5-[2,3]epipyranoanthra[2,3-f]isoindole
CAS name 15H-12,5[2',3']-endo-pyrano-1H-anthra[2,3-f]isoindole

The previous edition of these rules (ref 3) separated the indicated hydrogen needed for a bridge from that used for the fused ring system (see A-34.4 for example). This system is still used by CAS, who would also use it in, for example, 3a,6-ethano-3aH-indene but not with the bridge [2]pyrrolyl[5]ylidene (cf. rule B-5.12).

FR-9.4 Partially hydrogenated fused ring sysem

Certain partially hydrogenated fused ring systems are named separately (see Table 21 and 24 of ref 3a). These should not be used as components in fusion nomenclature. The following names are retained: chromane, indane, indoline, isochromane, isoindoline, selenochromane, isoselenochromane, tellurochromane, isotellurochromane, thiochromane, isothiochromane. In the previous edition of these rules (see rule A-23.1, ref 3) several other examples were listed i.e. acenaphthene, cholanthrene, aceanthrene, acephenanthrene, violanthrene, isoviolanthrene. These are no longer recommended. Beilstein uses all of these except indoline, isoindoline, violanthrene and isoviolanthrene. None are used by CAS. The system for naming partially hydrogenated hydrocarbon fused ring systems in rule A-23.5 (ref 3) is abandoned.

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.

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

19. IUPAC, Nomenclature for cyclic systems with contiguous formal double bonds (the delta convention), Pure Appl. Chem. 60, 1395-1401 (1988).

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