Additional Chirality Elements
Naming of the Absolute Configuration of Molecules with a Chirality Axis
The Cahn-Ingold-Prelog rules form the basis of the naming of absolute configuration in regard to molecules with a chirality axis. The molecule is orientated, so that the chirality axis is perpendicular to the illustration plane. Then the substituents that are responsible for the chirality are ordered according to the CIP rules, first the substituents in front (a, b) and then separately the substituents at the back (c, d), regardless of whether the substituent b shows a lower CIP priority than substituent c. In the illustration (III), the substituents a and b that are in front are connected by a wide line and the substituents c and d at the back are joined by a thin line.
If the sequence a - b - c - d of the substituents is clockwise, the absolute configuration is called (aR). If it is counterclockwise, the absolute configuration is called (aS). In the illustration above, the CIP priority of the nitro group is higher than that of the carboxyl group.