3.13: Disubstituted Cyclohexanes: cis-trans Isomerism
Depending upon the different spatial orientation of the substituents, the disubstituted cycloalkanes exhibit two types of stereoisomers. The cis isomers have the substituents on the same side of the ring, whereas the trans isomers have the substituents on the opposite sides. These stereoisomers exhibit different physical properties and cannot be interconverted without breaking the carbon-carbon bonds.
In cyclohexane, the substituents can occupy different positions generating distinct isomers. For instance, in the case of 1,4-dimethylcyclohexane, the cis isomer has two conformers equilibrating in equal proportions. Both conformers have one methyl group in the axial and the other in the equatorial positions. However, the conformers of the trans isomer are energetically non-equivalent. The trans conformer having both the methyl groups in the axial position experiences strong steric repulsion and is less stable than the diequatorial form. Hence the trans-diequatorial form is most abundant at equilibrium.
The isomers of 1,2-dimethylcyclohexane are similar to that of 1,4 counterparts. The trans-1,2-dimethylcyclohexane has two energetically non-equivalent conformers. Due to the absence of 1,3-diaxial interactions, the diequatorial form is more stable and abundant than the diaxial conformer. In contrast, both conformers of the cis-isomer have equivalent energies with one axial and one equatorial methyl group.
The presence of bulky substituents alters the equilibrium because the bulky substituent preferentially occupies the equatorial position to avoid the strong steric repulsion in the axial position.