Question

Consider diequatorial trans-1,2-dimethylcyclohexane and diaxial trans-1,2-dimethylcyclohexane as shown in the figure below [1, p. 178]. In both compounds, the ring is free of angle strain.

trans-1,2-Dimethylcyclohexane has a conformation in which both methyl groups are equatorial and one in which both methyl groups are axial. As we would expect, the conformation with both methyl groups equatorial is the more stable one.

It can be clearly seen from the figure that in the diaxial, the methyl groups are much farther away than they are in the diequatorial. Hence, the diaxial conformer should be more stable due to less torsional strain or less repulsive dispersion forces. But my book states the opposite. Why?

Reference
Solomons, T. W. G.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry, 11th ed.; Wiley: Hoboken, NJ, 2013. ISBN 978-1-118-13357-6.

Answer 1

The diequatorial conformation of trans-1,2-dimethylcyclohexane is more stable due to the absence of 1,3-diaxial interactions, which cause sterics strain in the diaxial conformation. This overrides the apparent distance between axial methyl groups, making the equatorial conformation energetically preferred.

Step-by-step explanation:

The conformation with both methyl groups equatorial in trans-1,2-dimethylcyclohexane is more stable despite the appearance that the methyl groups in the diaxial conformation are farther apart. The enhanced stability of the diequatorial conformer is due to 1,3-diaxial interactions, a type of steric strain that occurs when substituents at the axial positions interact with the axial hydrogen atoms on the same face of the cyclohexane ring.

In contrast, transannular strain, which refers to steric repulsion between groups on non-adjacent atoms within larger rings, plays a less significant role in the six-membered cyclohexane due to the optimal bond angles and lack of significant angle strain in its chair conformation.