Question

Question: Draw a stepwise mechanism for the following reaction, which results in ring expansion of a six-membered ring to a seven-membered ring.

Short answer

Answer

Step-by-step solution

Generation of carbocation  

The carbocations generated by the addition of acids to alkenes may undergo shifting (1,2-hydrogen or 1,2-methyl), depending upon their stability.

The stability order of the carbocations is$3°>2°>1°$ .

1,2-hydrogen and methyl shift

The stability of carbocations enables 1,2-hydrogen and methyl shift. After the shifting of methyl and hydrogen to adjacent positions, which is the most stable carbocation.

Explanation

The reaction of the given compound with water and sulfuric acid proceeds via 1,2-bond shift to generate a carbocation. The carbocation gives a tertiary alcohol product.

The mechanism is shown below:

Reaction mechanism