Question

a. Calculate the energy difference between the two chair conformers of trans-1,4-dimethylcyclohexane.

b. What is the energy difference between the two chair conformers of cis-1,4-dimethylcyclohexane?

Short answer

a)The energy difference between the two chair conformers of trans-1,4-dimethylcyclohexaneis 3.6 Kcal/mol.

b) Two conformers are equally stable.

Step-by-step solution

Subpart (a) The energy difference between the two chair conformers of trans-1,4-dimethylcyclohexane.

In one conformer both methyl groups are in equatorial position so there will be no strain. In another conformer, both methyl groups are in axial position so there will be two 1,3 diaxial interactions so total four 1,3 diaxial interactions are there.

Methyl group and hydrogen causes strain energy is 0.9Kcal/mol.

Four 1,3 diaxial interactions is 4 x 0.9 = 3.6 Kcal/mol.

The chair conformer with both substituents in the axial position will be 3.6Kcal/mol less stable than the chair conformer with both substituents in equatorial positions.

Subpart (b) The energy difference between the two chair conformers of cis-1,4-dimethylcyclohexane.

Here, two conformers are equally stable as one substituent is in the equatorial position and one substituent is at the axial position.