Question

On treatment with HBr, vinylcyclohexane undergoes addition and rearrangement to yield 1-bromo-1-ethylcyclohexane. Using curved arrows, propose a mechanism to account for this result.

Short answer

If the electrophilic addition reaction happens in a single step, it is difficult to explain the rearrangement reaction. Those rearrangement reactions happening in multiple steps can be explained easily.

Step-by-step solution

Step-by-Step solutionStep 1: Electrophilic addition: Carbocation rearrangement

If the electrophilic addition reaction happens in a single step, it is difficult to explain the rearrangement reaction. Those rearrangement reactions happening in multiple steps can be explained easily.

Mechanism for the reaction of vinyl cyclohexane with HBr

In the first step, the double bond of the vinylcyclohexane attacks the HBr to yield a secondary carbocation intermediate, expelling a bromide ion. In the next step, the secondary carbocation generated rearranges to a tertiary carbocation by a hydride shift from the adjacent carbon. In the final step, the bromide ion reacts with the tertiary carbocation to generate 1-bromo-1-ethylcyclohexane.

The mechanism can be given as:

Mechanism of the reaction of vinylcyclohexane with HBr