Question

Question: The heat of combustion of cis-1,2-dimethylcyclopropane is larger than that of trans isomer. Which isomer is most stable? Use drawings to explain this difference in stability.

Short answer

Answer

Between cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane, trans-1,2-dimethylcyclopropane is more stable.

Step-by-step solution

Cis and trans isomers

Both cis- and trans-isomers are geometrical isomers but differ in their spatial arrangement. In cis-isomer same functional group (or element or group of atoms) is on the same side while in the trans-isomers same functional group (or element or group of atoms) is on the opposite side of each other.

Explanation of the more stable isomer

Here,trans-1,2-dimethylcyclopropane is more stable because in cis-isomer two methyl groups are either above or below the cyclopropane ring causing steric hindrance which makes cis isomer less stable (shown in figure). On the other hand, in the case of trans-isomer, both methyl groups are opposite to each other causing less steric hindrance resulting in higher stability of trans-isomer (shown in figure).