Question

cis-1, 2-Dimethylcyclobutane is less stable than its trans isomer, but cis-1, 3-dimethylcyclobutane is more stable than its trans isomer. Draw the most stable conformations of both, and explain.

Short answer

Cis-1, 2-Dimethylcyclobutane is less stable than its trans isomer because of eclipsed strain observed in cis-1, 2-dimethylcyclobutane.

Step-by-step solution

Introduction

Structures of trans and cis forms of 1, 2 dimethylcyclobutane are as shown below:

Cis-1,2-dimethylcyclobutane

Trans-1,2-dimethylcyclobutane

Stability of trans and cis form of 1, 2 dimethylcyclobutane

When we observed trans-1, 2-dimethylcyclobutane, both the methyl groups occupied an equatorial position.

In the case of cis-1, 2-dimethylcyclobutane, one methyl group is present in the axial position while the other methyl group is present in the equatorial position. So 1, 3 diaxial interaction or eclipsed strain is observed in the case of cis-isomer so it is less stable than the trans-1, 2-Dimethylcyclobutane.

Trans and cis form of 1, 3- dimethylcyclobutane

Structures of trans and cis forms of 1, 3- dimethylcyclobutane are as shown below:

Stability of trans and cis form of 1, 3- dimethylcyclobutane

In the case ofcis-1, 3-Dimethylcyclobutane both the methyl groups present in an equatorial position. While in the case of trans-1, 3-Dimethylcyclobutane one methyl group is present in equatorial while the other one is present in axial position. So in trans-1, 3-Dimethylcyclobutane 1, 3 diaxial interactions are observed. Hence cis-1, 3-dimethylcyclobutane is more stable than its trans isomer.