Question

Propose mechanisms to account for the observed products in the following reactions.

Short answer

(a)

(b)

Step-by-step solution

Allylic substitution

In allylic bromination, the Br atom appears on the carbon next to the double bond which proceeds via a radical mechanism. The allylic radicals are very stable because of resonance-stabilization.

Proposing the mechanism

Here, NBS will create a radical on the allylic position of the double bond and one more isomer will be formed by the rearrangement of the radical formed. The Br will be attached to

the radical position and will lead to the final product formation. Below are the mechanisms proposed for the given two parts:

(a)

Mechanism for reaction (a)

(b)

Mechanism for reaction (b)