Q7-21P - Organic Chemistry (Mcmurry)

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.

Step-by-Step Solution

Answer

Step-by-Step solution Step 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.

Step 2: 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