Question

trans-1,4-Di-tert-butylcyclohexane exists in a normal chair conformation. cis-1,4-Di-tertbutylcyclohexane, however, adopts a twist-boat conformation. Draw both isomers and explain why the cis isomer is more stable in a twist- boat conformation.

Short answer

Answer: The cis isomer of 1,4-di-tert-butylcyclohexane is more stable in a twist-boat conformation because it reduces the steric strain and repulsion between the large tert-butyl groups compared to the chair conformation. The twist-boat conformation allows the tert-butyl groups to be further apart from each other, leading to greater stability.

Step-by-step solution

Draw the chair conformation of trans-1,4-di-tert-butylcyclohexane

Draw a chair conformation of cyclohexane with three carbon atoms above the plane (up-equatorial) and three carbon atoms below the plane (down-equatorial). Place the tert-butyl groups on the 1 and 4 positions, with one pointing up and the other pointing down, giving you the trans configuration.

Draw the twist-boat conformation of cis-1,4-di-tert-butylcyclohexane

For the cis isomer, draw a twist-boat conformation of the cyclohexane ring with 1 and 4 carbon atoms having the tert-butyl groups pointing in the same direction. This means both tert-butyl groups will be pointing up (or down).

Explain the stability of the cis isomer in twist-boat conformation

The steric strain due to the large tert-butyl groups in the cis isomer is too strong to maintain the chair conformation creating unfavorable repulsion and unstable conformation. When the cis isomer is in a twist-boat conformation, there is less steric strain and repulsion between the groups. The twist-boat conformation allows the two tert-butyl groups to be further apart from each other reducing repulsion and leading to greater stability.