Fused Ring and Bridged Fused Ring Nomenclature
(IUPAC Recommendations 1998)

FR-5.3 Peripheral.Numbering

Continued from FR-5.2 Orientation

Contents of Section

FR-5.3 Peripheral numbering
FR-5.4 Order of preference between alternative numbering
FR-5.5 Interior numbering
FR-5.5.1 Heteroatoms
FR-5.5.2 Carbon atoms Reference for this Section

Continue with FR-6 Multi-Parent Systems

FR-5.3 Peripheral numbering

The numbering of the peripheral atoms in a particular 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 non-fusion atom most counterclockwise in the ring selected, and numbering proceeds in a clockwise direction around the system, including fusion heteroatoms. Fusion carbon atoms are not included; each fusion carbon atom is given the same number as the immediately preceding position modified by a roman letter a, b, c, etc. If heteroatoms are identified by replacement nomenclature all atoms retain the numbers of the parent hydrocarbon.





Polyhelicenes are treated differently as an exception to this rule (see FR-2.1.7).

If there are rings of different sizes consideration of the uppermost ring is based on the level above the horizontal row of FR-5.2.a in the idealised two dimensional hexagonal grid (see start of FR-5) rather than the geometric height above the horizontal row. Rings directly fused to the horizontal row identified in FR- 5.2.a are considered as the first row above the horizontal. The second row above are fused to the first row and so on.


(The indene six-membered ring is two rows above the horizontal row.)
(The ten-membered ring is only one row above the horizontal row.)

If due to overlap there is still a choice the ring fused to the ring furthest right is chosen.




The distortion here is introduced only in order to see each ring clearly.

If there is a choice of starting points numbering starts from the atom most counterclockwise in a continuous non-fusion portion of the ring selected. If there is still a choice of starting points the uppermost one is selected.


(nubering does not start at position 15)

If the selected ring does not have a non-fusion atom then numbering starts in the next ring encountered when proceeding round the system in a clockwise direction.





Cyclopenta[a]phenanthrene is numbered as a steroid (ref 12) by CAS. In these recommendations it is numbered in the standard way. This is a change from rule A-22.5 (ref 3).

CAS also names and numbers additional cyclopropane rings fused to cyclopenta[a]phenanthrene using this numbering e.g. cyclopropa[2,3]cyclopenta[a]phenanthrene instead of cyclopenta[a]cyclpropa[h]phenanthrene. This numbering is also used for oxirene and thiirene rings (but not azirine) fused to cyclopenta[a]phenanthrene. These rings are indicated by CAS using the corresponding bridging prefixes (epoxy and epithio).

FR-5.4 Order of preference between alternative numbering systems

If there are alternative orientations of a ring system which are equally preferred after the application of FR- 5.2 (including alternative locations for the heteroatoms) then the following rules are applied in order until a preferred numbering of the peripheral atoms is identified.

a. Give low numbers for the heteroatoms as a set.


(1,3,4 is preferred to 1,3,6 or 1,4,6)

b. Give low numbers for heteroatoms when considered in the order: O, S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn, Pb, B, Hg [i.e. Table I of the revised Hantzsch-Widman system (ref 9)].




c. Give low numbers to fusion carbon atoms.


not nor
(4a is lower than 8a)

(2a,5a,9a is lower than 2a,6a,9a; 3a,5a,9a; 3a,7a,9a; 4a,6a,9a; or 4a,7a,9a)

(3a,3b,3c... preferred to 3a,4a,5a...) error details

d. Give low numbers to fusion rather than non-fusion atoms of the same heteroelement.


(5 is preferred to 6)

e. Give low numbers so that an internal heteroatom is nearer (i.e. less bonds in the pathway) to the lowest numbered fusion peripheral atom.


(11 is 1 bond from 3a)
CAS name 6H-benzo[ij]pyrido[2,1,6-de]quinolizine)


In this example the peripheral numbering is determined by the nitrogen atom not by indicated hydrogen.

(13 is 1 bond from 2a)

f. Give low numbers to indicated hydrogen atoms (expressed or implied).


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

not 3H-cyclopenta[l]phenanthrene

FR-5.5 Interior numbering

FR-5.5.1 Heteroatoms

Interior heteroatoms (not identified by replacement nomenclature) are numbered after the peripheral atoms continuing the number sequence. If there is a choice the shortest path of bonds from each heteroatom to the periphery is determined. The lower number is given to the heteroatom which is closest to the lowest numbered peripheral atom.



(9 is one bond from 2a)

(nitrogen has a lower number than phosphorus, see FR-5.4.b)


Previously it was recommended that interior heteroatoms were numbered last following the shortest path from the highest previous number (rule B-3.4.e ref 3).

FR-5.5.2 Carbon atoms

Interior carbon atoms are numbered by identifying the minimum number of bonds linking them to the periphery. 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.




Rule A-22.2 recommended that interior atoms follow the highest numbered peripheral atom sequence. For example those in pyrene (below) were numbered 10b and 10c (3a1 and 5a1 respectively). CAS and Beilstein still use this system.

If there is a choice of interior atom locants lower numbers are preferred.







CAS name 1H,5H-benzo[ij]quinolizine


A non-free spiro peripheral atom is not used as reference for interior carbon atoms, if ambiguity would result. In the first example below carbon atom 3a is a non-free spiro peripheral atom.





If 3a had been used as a reference atom in the first example both 6a1 and 16a1 would be 3a1.

The system described in A-22.2 (ref 3) has been developed by CAS to deal with complex examples (See ref 18). Their additional rules to number interior atoms for complex examples are as follows.

a. Starting from the highest numbered atom (peripheral atom or interior numbered heteroatom) the longest continuous path of interior atoms is determined and numbered by adding the letter 'a', 'b', 'c', etc. in turn to that of the highest numbered atom. This procedure is repeated at the next highest numbered atom etc. until all interior atoms are numbered.

b. If there are alternative pathways the two alternative atoms at the branch point are compared using the shortest path to the periphery for each atom. The atom linked in this way to the lower numbered peripheral atom is selected.

c. Isolated interior atoms and chains not covered by (a) or (b) above are numbered using the longest chain of unnumbered atoms with the numbering continuing from the highest numbered alphanumeric locant available.

d. If there are interior numbered heteroatoms the longest chain of interior atoms attached to the interior heteroatom is numbered as above.






Reference for this Section

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.

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.

12. IUPAC-IUB, Nomenclature of steroids, Eur. J. Biochem. 186, 429-458 (1989); Pure Appl. Chem. 61, 1784-1822 (1989).

18. C.L. Gladys and A.L. Goodson, Numbering of interior atoms in fused ring systems, J. Chem. Inf. Comput. Sci. 31, 523-526 (1991).

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