Question

Draw the most stable conformation of

(a) ethylcyclohexane

(b) 3-isopropyl-1,1-dimethylcyclohexane

(c) cis - 1-tert- butyl - 4 - isopropylcyclohexane

Short answer

(a)

equatorial ethyl group (more stable)

(b)

equatorial isopropyl group (more stable)

(c)

equatorial tert butyl and axial isopropyl group (more stable)

Step-by-step solution

Axial and equatorial positions of cyclohexane

Axial bonds are directed vertically parallel to the axis of the ring and the equatorial bonds are directed outward from the equator of the ring. As the carbon atoms are numbered in cyclohexane, it can be seen that the odd numbered carbons have their upward bonds in axial position and their downward bonds in equatorial position. The even numbered carbons have their downward bond sin axial position and their upward bonds in equatorial position.

Identifying the most stable conformation

Large substituents tend to be in equatorial position.

(a)

equatorial ethyl group (more stable)

In ethylcyclohexane , most stable conformation is the one in which the ethyl group is present in equatorial position.

(b)

equatorial isopropyl group (more stable)

In 3-isopropyl-1,1-dimethylcyclohexane , the most stable conformation is the one in which the isopropyl group is present in equatorial position.

(c)

equatorial tert butyl and axial isopropyl group (more stable)

In cis - 1-tert- butyl - 4 - isopropylcyclohexane , most stable conformation is the one in which the tert butyl group is present in equatorial position and the isopropyl group is present in axial position. tert butyl group is in equatorial position since it is bulkier than the isopropyl group.