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.
Table showing axial-equatorial energy differences for various groups in cyclohexane.

Table showing axial-equatorial energy differences for various groups in cyclohexane.

Accepted Answer

To draw the axial conformation of isopropylcyclohexane, start by sketching a cyclohexane chair conformation. Place the isopropyl group on one of the axial positions, which are perpendicular to the plane of the ring. For tert-butylcyclohexane, similarly draw a cyclohexane chair conformation and place the tert-butyl group in an axial position. The tert-butyl group is larger and bulkier than the isopropyl group. The axial position in a cyclohexane chair conformation is less stable for larger groups due to steric hindrance and 1,3-diaxial interactions. These interactions occur between the axial substituent and the hydrogen atoms on the same side of the ring. The tert-butyl group experiences a larger increase in axial energy compared to the isopropyl group because it is bulkier, leading to greater steric hindrance and more significant 1,3-diaxial interactions. The energy difference between axial and equatorial positions is much larger for tert-butyl groups because the equatorial position allows the bulky group to avoid steric clashes, making it significantly more stable than the axial position.

비슷한 문제에 대한 검증된 영상 답변:

주요 개념

Axial and Equatorial Positions

Steric Hindrance

Energy Differences in Conformations