Question

Table 3-6 shows that the axial-equatorial energy difference for methyl, ethyl, and isopropyl groups increases gradually: 7.6, 7.9 and 8.8 kJ/mol (1.8, 1.9, and 2.1 kcal/mol). The tert-butyl group jumps to an energy difference of 23 kJ/mol (5.4 kcal/mol), over twice the value for the isopropyl group. Draw pictures of the axial conformations of isopropylcyclohexane and tert-butylcyclohexane and explain why the tert-butyl substituent experiences such a large increase in axial energy over the isopropyl group.

Short answer

Isopropylcyclohexane tert-butylcyclohexane

The tertiary butyl group in tert-butyl cyclohexane causes hindrance and methyl group of tert-butyl group shows interactions with hydrogens as shown, thus resulting in high energy of the conformer.

Step-by-step solution

Step-1. Axial conformations of isopropylcyclohexane and tert-butylcyclohexane:

Cyclohexane has axial and equatorial bonds. Equatorial bonds are located at the periphery of cyclohexane ring in chair conformation whereas axial bonds are oriented above and below plane of ring. In axial conformation, the isopropyl group in isopropylcyclohexane will be at the axial position whereas in tert-butylcyclohexane, tert-butyl group will be at the axial position.

Isopropylcyclohexane tert-butylcyclohexane

Step-2. Higher axial energy in case of tert-butylcyclohexane:

The isopropyl group in isopropylcyclohexane can rotate due to less steric hindrance so there will be no repulsion between axial hydrogens and hydrogen of isopropyl group whereas tert-butyl group has methyl group which causes repulsion due to its closeness with axial hydrogens and also it cannot rotate due to it’s large size, thus the stability of the system decreases and energy of the conformer increases. Thus, axial conformer of tert-butylcyclohexane has higher energy than axial conformer of isopropylcyclohexane.