Stability of Substituted Cyclohexanes

The two chair conformations of cyclohexane, at equilibrium, have identical energies and stabilities, with each conformer representing about 50% of the equilibrium mixture.

Replacing a hydrogen atom with an alkyl group makes the two conformations energetically nonequivalent.

For instance, in methylcyclohexane, the CH₃ group occupies an axial position in one chair conformation and an equatorial position in another.

The axial conformation has an elevated energy of approximately 7.6 kJ mol⁻¹ compared to the equatorial conformation, making the latter more stable.

Consequently, the equatorial conformation comprises about 95% of the equilibrium mixture. The question is — what is the reason for such variations in energy and stability?

Studies reveal that in an axial conformation, the methyl hydrogens experience repulsive dispersion interactions with the two parallel and closely positioned axial hydrogens on the same side of the ring.

This unfavorable steric strain between groups on C1 and C3 or C5 is called 1,3-diaxial interaction, which is a gauche interaction.

Each gauche interaction contributes approximately 3.8 kJ of additional energy.

Such gauche interactions are absent in an equatori