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Acyclic Hydrocarbons

Rule A-2. Saturated Branched-chain Compounds and Univalent Radicals

2.1 - A saturated branched acyclic hydrocarbon is named by prefixing the designations of the side chains to the name of the longest chain present in the formula.

Example to Rule A-2.1

The following names are retained for unsubstituted hydrocarbons only:

2.2 - The longest chain is numbered from one end to the other by Arabic numerals, the direction being so chosen as to give the lowest numbers possible to the side chains. When series of locants containing the same number of terms are compared term by term, that series is "lowest" which contains the lowest number on the occasion of the first difference. This principle is applied irrespective of the nature of the substituents.

Examples to Rule A-2.2

2.25 - Univalent branched radicals derived from alkanes are named by prefixing the designation of the side chains to the name of the unbranched alkyl radical possessing the longest possible chain starting from the carbon atom with the free valence, the said atom being numbered as 1.

Examples to Rule A-2.25


The following names may be used for the unsubstituted radicals only:

2.3 - If two or more side chains of different nature are present, they are cited in alphabetical order .

The alphabetical order is decided as follows:

(i) The names of simple radicals are first alphabetized and the multiplying prefixes are then inserted.

Examples to Rule A-2.3(i)

ethyl is cited before methyl,
thus 4-Ethyl-3,3-dimethylheptane

(ii) The name of a complex radical is considered to begin with the first letter of its complete name.

Examples to Rule A-2.3(ii)

dimethylpentyl (as complete single substituent) is alphabetized under "d",
thus 7-(1,2-Dimethylpentyl)-5-ethyltridecane

(iii) In cases where names of complex radicals are composed of identical words, priority for citation is given to that radical which contains the lowest locant at the first cited point of difference in the radical.

Examples to Rule A-2.3(iii)

2.4 - If two or more side chains are in equivalent positions, the one to be assigned the lower number is that cited first in the name.

Examples: to Rule A-2.4

2.5 - The presence of identical unsubstituted radicals is indicated by the appropriate multiplying prefix di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, undeca, etc.

Example to Rule A-2.5(a)

The presence of identical radicals each substituted in the same way may be indicated by the appropriate multiplying prefix bis-, tris-, tetrakis-, pentakis-, etc. The complete expression denoting such a side chain may be enclosed in parentheses or the carbon atoms in side chains may be indicated by primed numbers.

Examples to Rule A-2.5(b)

(a) Use of parentheses and unprimed numbers:


(b) Use of primes:


(a) Use of parentheses and unprimed numbers:


(b) Use of primes:


2.6 - If chains of equal length are competing for selection as main chain in a saturated branched acyclic hydrocarbon, then the choice goes in series to:

(a) The chain which has the greatest number of side chains.

Example to Rule A-2.6(a)

(b) The chain whose side chains have the lowest-numbered locants.

Example to Rule A-2.6(b)

(c) The chain having the greatest number of carbon atoms in the smaller side chains.

Example to Rule A-2.6(c)

(d) The chain having the least branched side chains.

Example to Rule A-2.6(d)

Unsaturated Compounds and Univalent Radicals
Bivalent and Multivalent Radicals

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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