Question

Reaction of HBr with 3-methylcyclohexene yields a mixture of four products: cis- and trans-1-bromo-3-methylcyclohexane and cis- and trans-1-bromo-2-methylcyclohexane. The analogous reaction of HBr with 3-bromocyclohexene yields trans-1,2-dibromocyclohexane as the sole product. Draw structures of the possible intermediates, and then explain why only a single product is formed in this reaction.

Short answer

Reaction of HBr with 3-methylcyclohexene forms two products due to the formation of the two carbocation, and the stability of both the carbocation is almost equal.So, this leads to 50% chance of both the products forming, and it is shown below.

Step-by-step solution

Reaction of HBr with 3-methylcyclohexene

Reaction of HBr with 3-methylcyclohexene forms two products due to the formation of the two carbocation, and the stability of both the carbocation is almost equal.So, this leads to 50% chance of both the products forming, and it is shown below.

Reaction of HBr with 3-bromocyclohexene

The reaction of HBr with 3-bromolcyclohexene forms single products due to the formation of the two carbocations, but the stability of one the carbocation is very high from the other. So, this leads to 100% chance of a single product being formed, and it is shown below.