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Substitutive Nomenclature

Numbering of Compounds Rule C-15.1

15.11 - In so far as Sections A and B of the I.U.P.A.C. rules leave a choice, the starting point and direction of numbering of a compound are chosen so as to give lowest locants to the following structural factors (if present), considered successively in the order listed until a decision is reached; however principal groups take precedence over multiple bonds.

(a) Indicated hydrogen (whether cited in the name or omitted as being conventional).

Examples to Rule C-15.11(a)

(b) Principal groups named as suffix.

Examples to Rule C-15.11(b)

(c) Multiple bonds in acyclic compounds (see Rules A-3.1 to A-3.4), in cycloalkanes (see Rule A-11), and in bi-, tri-, and poly-cycloalkanes (see Rules A-31 and A-32), double having priority over triple bonds, and in heterocyclic systems whose names end in -ethine, -oline, or -olene (see Rules B-1.1 and B-1.2).

Examples to Rule C-15.11(c)

(d) Lowest locants for substituents named as prefixes, hydro prefixes, -ene and -yne, all considered together in one series in ascending numerical order.

Examples to Rule C-15.11(d)

(e) Lowest locant for that substituent named as prefix which is cited first in the name (see Subsection C-0.16).

Examples to Rule C-15.11(e)

Note: Hydro and dehydro prefixes, if treated as detachable (see Rule C-16.11), are considered along with prefixes for substituents when this criterion is applied.

Numbering of Compounds
Rule C-15.2


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.

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