Allylic Bromination [NBS]

Allylic Bromination:

The substitution reaction in which bromination occurs at the allylic carbon is called as allylic bromination.

The carbon attached to an sp2 carbon is called allylic carbon.

The general reaction of allylic bromination is given as;

The reaction takes place in the presence of NBS which results in controlled bromination at the allylic position.

Mechanism:

The reaction proceeds through the free radical mechanism. The first step (Initiation) involves the splitting N-Br bond by homolytic cleavage. It results in the formation of Br free radical.

The succinimide radical formed is resonance stabilized, due to this stabilization the homolysis of the N-Br bond and formation of bromine radical become possible.

In the next step, the bromine free radical capture hydrogen from the allylic carbon, and allylic free radical is formed. The allylic free radical is resonance stabilized. This stabilization of allylic radical is responsible for allylic bromination. Simple halogenation does not take place because of this stability.

Then another molecule of NBS reacts with the HBr and Br₂ is produced. This Br₂ molecule undergoes homolytic cleavage again and Bromine free radical reacts with allylic free radical resulting in the formation of allylic bromide product. As both allylic free radicals are identical only one product is formed.

That is how NBS produce Br2 in lower concentration which resulted in controlled bromination.

 For this reactant, only one product can be formed.

In other alkenes containing more than 3 carbons, a mixture of products is formed. Because different free radicals are formed due to resonance stabilization. The resultant products are constitutional isomers.

 

Consider the following reaction:

The allylic bromination reaction is not stereo-chemically controlled. All isomers can form. Therefore it results in the formation of enantiomers.