Heterocyclic Systems

Rule B-4. Replacement Nomenclature (also known as "a" Nomenclature)

4.1 - Names of monocyclic hetero compounds may be formed by prefixing "a" terms (see Table B-I), preceded by their locants, to the name of the corresponding hydrocarbon. Numbering is assigned so as to give locant 1 to the hetero atom listed earlier in Table B-I, next to hetero atoms as a complete set, and then to hetero atoms in order of Table B-I. If a further choice is possible, numbering is assigned according to Rule C-0.15.

Examples to Rule B-4.1





4.2 - Fused heterocyclic systems may be named by prefixing "a" terms preceded by their locants, to the name of the corresponding hydrocarbon. The numbering of the corresponding hydrocarbon is retained, irrespective of the position of the hetero atoms; where there is a choice, low numbers are assigned in the following order: first to hetero atoms as a complete set, next to hetero atoms in order of Table B-I, and then to multiple bonds in the heterocyclic compound according to the principles of Rule A-11.3. These principles are applied in one of two ways, as follows:

(a) When the corresponding hydrocarbon does not contain the maximum number of non-cumulative double bonds and can be named without the use of hydro prefixes, as for indan, then the hydrocarbon is named in that state of hydrogenation.

Example to Rule B-4.2(a)


(b) When the two conditions of paragraph (a) are not fulfilled, positions in the skeleton of the corresponding hydrocarbon that are occupied by hetero atoms are denoted by "a" prefixes, and the parent heterocyclic compound is considered to be that which contains the maximum number of conjugated or isolatedfront of the "a" terms.





4.3 - In fusion names, the "a" terms precede the completed name of the parent hydrocarbon. Prefixes denoting ordinary substitution precede the "a" terms.

Examples to Rule B-4.3


See Recommendations'93 R-9.3

Cationic Hetero Atoms


This HTML reproduction of Sections A, B and C of IUPAC "Blue Book" is as close as possible to the published version [see Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, Pergamon Press, Oxford, 1979. Copyright 1979 IUPAC.] If you need to cite these rules please quote this reference as their source.

Published with permission of the IUPAC by Advanced Chemistry Development, Inc.,, +1(416)368-3435 tel, +1(416)368-5596 fax. For comments or suggestions please contact