Question

Draw the structure of the major product formed in the following reaction of p-cymene with N-bromosuccinimide under the conditions shown. The molecular formula of the product is given.

Answer 1

Concepts and reason

Addition of bromine to an substrate is called as bromination, It can be added across the unsaturated bonds or substituted in the saturated systems.

Allylic bromination is the replacement of a hydrogen atom by a bromine atom at the allylic position. The allylic position is an adjacent carbon position to the double bond. NBS (N-Bromo Succinimide) is used for brominating at allylic positions in the presence of peroxides, light or heat.

Free radical reaction is a type of reaction which involves free radical. It can either be free radical substitution or free radical addition.

Fundamentals

• Allylic bromination undergoes a free radical mechanism by NBS in the presence of peroxides and light or heat.

• NBS is catalyzed by ${\rm{HBr}}$ , which is present in NBS as a small quantity; that is why the direct bromination of a double bond is controlled. ${\rm{CC}}{{\rm{l}}_{\rm{4}}}$ acts as a solvent in this reaction

• This reaction undergoes a radical chain reaction, which mainly has three steps

I.Initiation

II.Propagation

III.Termination

The reaction is represented as shown below.

Propagation (1)

Propagation (2)

Termination

Ans:

The major product obtained from the NBS bromination of p-cymene was given below

Answer 2

Sol :-

N-Bromosuccinimide (i.e. NBS) is specially used for the preparation of allylic or benzylic bromide. It is a type of allylic or benzylic bromination which obey free radical mechanism in the presence of peroxide. We know that tertiary benzylic free radical is more stable than primary benzylic free radical, therefore tertiary benzylic brominated products will be the major products.