Stereoisomers of Cyclic Compounds
The interconversion of a cyclohexane ring from one chair conformation into the other leads to the exchange of axial and equatorial positions. However, the cis/trans relationship of the substituents is not altered by any conformational changes of cycloalkanes.
Look at 1,4-dimethylcyclohexane as an example. 1,4-Dimethylcyclohexane consists of two diastereomeric stereoisomers, although, according to the n2 rule, four stereoisomers should be expected due to the fact that 1,4-dimethylcyclohexane contains two asymmetric carbons. However, as cis- and trans-1,4-dimethylcyclohexane contain an internal symmetry plane, only two stereoisomers appear. Therefore, they are both meso compounds.
In contrast to 1,4-dimethylcyclohexane, only cis-1,3-dimethylcyclohexane contains an internal symmetry plane, while trans-1,3-dimethylcyclohexane does not. As a result, only three stereoisomers of 1,3-dimethylcyclohexane are observed: cis-1,3-dimethylcyclohexane, which is a meso compound, and a pair of trans-1,3-dimethylcyclohexane enantiomers.
The relative configuration of the cis isomer is (1R*,3S*) while the relative configuration of the trans isomer is (1R*,3R*). Since the absolute configuration of any chirality center is not transformed by conformational alterations, even though axial substituents change into equatorial substituents and vice versa, the trans-1,3-dimethylcyclohexane enantiomers cannot be interconverted by a ring flip.
The three-dimensional structures of the trans isomers of 1,3-dimethylcyclohexane and the ring flip of trans-(1S,3S)-1,3-dimethylcyclohexane are illustrated by the interactive molecular models and animation below.
In order for the cyclohexane conformations to be clearly viewed, the hydrogens are not shown in the interactive three-dimensional molecular models (Chime plugin). Additionally, the axial methyl groups are coloured red and the equatorial methyl groups are coloured blue.
Enantiomers I and II are nonsuperimposable. Axial and equatorial positions are exchanged by the ring flip. The absolute configurations of the chirality centers are not changed by the ring flip. Examine the stereochemical characteristics of trans-1,3-dimethylcyclohexane by manipulating the interactive molecular models of enantiomers I and II with your mouse.