Equatorial Methylcyclohexane (larger view)

Axial Methylcyclohexane (larger view)

(How to manipulate JSmol structures)

Methylcyclohexane

Methylcyclohexane contains a plane of symmetry that passes through the methyl group, the methine carbon (C₁) to which the methyl group is attached, and C₄. The red carbon atoms in the equatorial conformation form an anti-butane conformation. [For a review of the anti-butane conformation go back to butane.] There is also a mirror image anti-butane conformation on the other side of the plane. The equatorial conformation is taken as 0 kcal/mol. The axial conformation has a gauche butane interaction [Review: butane] of the methyl substituent with the ring as shown by the red carbon atoms. There is a second gauche butane interaction on the mirror side of the plane. Thus, axial methylcyclohexane is 2 x 0.9 kcal/mol = 1.8 kcal/mol less stable than the equatorial conformation at 298 ^oK. [Some sources give this value as 1.7 kcal/mol.] View the red atoms as a Newman projection and locate the 60^o dihedral angle. [If you measure the dihedral angle it is 72^o, and not 60^o. The cyclohexane ring does not have the degree of freedom that an acyclic chain has.]