18E-b - Organic Chemistry (Mcmurry)

Question

Draw the electron-pushing mechanism for the propagation steps of the allylic bromination reactions below. You may omit NBS in your mechanism, and use Br∙ and Br₂.

(b)

Step-by-Step Solution

Answer

The propagation step of allylic bromination using NBS is given below:

Step 1: Allylic/Benzylic bromination

It is the replacement of a hydrogen atom on a carbon atom adjacent to a double bond. 

NBS is used as a substitute for Br, as it provides very low level concentration of Br, and the reaction takes place without any competitor. NBS reacts in the presence of light.

 

As a result, a species with an unpaired electron is formed, which is highly reactive, unstable and short lived. This species is known as free radical.

Due to breaking of double bond, the bromine atom becomes attached to the carbon atom. It is a three step mechanism.

• Initiation
• Propogation
• Termination

Step 2: Initiation

During initiation, the weakest bond of N-Br in NBS cleaved homolytically, using the light energy and generation of bromine radical occurs.

Step 3: Propogation

In this step, the bromine radical replaces the hydrogen from allylic carbon. An allylic radical is formed, which then reacts with Br₂ to produce desired product and regeneration of bromine radical takes place. The process is goes on, until the NBS is consumed completely.

Step 4: Termination

In this step, two radicals combine with each other to stop the chain reaction.