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Stereochemical Specification

R-7.2.1 The R/S convention

Chiral compounds in which the absolute configuration is known are differentiated by the stereodescriptors R and S assigned by the sequence-rule procedure (see also Section E in the 1979 edition of the IUPAC Nomenclature of Organic Chemistry, and R.S. Cahn, C.K. Ingold, and V. Prelog, Angew. Chem., 78, 413-447 (1966); and V. Prelog and G. Helmchen, Angew. Chem., Int. Ed., 21, 567-583 (1982)) and are preceded when necessary by appropriate locants. For carbon (or other atoms) to which four different ligands are attached in the system Cabcd may be depicted as in 1, where a>b>c>d.

If the model 1 is viewed from the side remote from the fourth member d, the path from a to b to c traces a clockwise direction, and the system has the 'R' configuration; if passing from a to b to c traces an anticlockwise direction, as in 2, the configuration is 'S'.

The sequence rule itself is the method by which the groups a, b, c, d are assigned priorities. It contains five sub-rules, but only the first two are mentioned here, the other sub-rules being needed in relatively few cases.

Sub-rule 1: Higher atomic number precedes lower.

Here, the atomic numbers of the atoms Br, Cl, C, H attached to the central carbon atom are arranged in that order by sub-rule 1, giving the model shown, which is in the R configuration.

When two or more atoms attached to the chiral centre are alike, the comparison is extended through each of these atoms to succeeding atoms until a sequence priority can be established on the same basis. Multiple bonds are considered as two or three single bonds to the same atom. Thus, a group is treated as

and as

where (O) and (C) are 'duplicate representations' of the respective atoms which, if needed for further comparison, are attached to 'phantom atoms' having neither atomic number nor mass. Since O is preferred to C when the sequences O, (O), H and C, (C), H are compared, the group has sequence rule priority over the .

Examples to R-7.2.1

Similarly, a group is treated as

Sub-rule 2: Higher mass number precedes lower (applies only when sub-rule 1 does not decide priority).

In this case, the mass numbers allow the atoms to be arranged in the order Cl, C, , , resulting in an R configuration.

If a molecule contains more than one chiral centre, this procedure is applied to each, and the configuration is expressed as a set or R,S symbols. In names of compounds, the symbols R and S, with locants if needed, are written in parentheses, followed by a hyphen, in front of the complete name or appropriate substituent.

Examples to R-7.2.1

R-7.2.2 Relative configuration

This HTML reproduction is as close as possible to the published version [see IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), 1993, Blackwell Scientific publications, Copyright 1993 IUPAC]. If you need to cite these rules please quote this reference as their source.

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