tert-Butylcyclohexane

The tert-butyl group has such a high conformational energy that it is difficult to measure it with precision. Considering the rationale used for its predecessors, it is easy to understand why.

In any arrangement around the C-C bond between ring's carbon 1 and the central carbon of the tert-butyl group, there will always be a methyl group of the tert-butyl pointing towards the ring center. That implies a severe invasion of the van der Waals radius of the ring's axial hydrogens at positions 3 and 5 by the methyl group. The steric interactions are thus extremely large in the axial conformation that prevents it from participating in the chair-chair equilibrium. Consider that a conformational energy of 5 kcal/mol corresponds at room temperature to a 99.98/0.02 equatorial/axial ratio at equilibrium.