Chapter 12, Problem 34P

Interpretation Introduction

Interpretation:

The mechanism for the given reaction has to be proposed.

Concept introduction:

Radical or free radical: unpaired valence electron of an atom, molecule, or ion is called as radical.

Bond strength is depends on the formation of the radical, if the radical is involving in resonance which is weakest bond strength.

Radical or free radical: unpaired valence electron of an atom, molecule, or ion is called as radical.

Bromination of Allylic Carbons:

N-bromosuccinimide (NBS) is used for the allylic bromination through radical reaction. bromination of  allylic carbon requires low concentration of bromine and low concentration of hydrobromic acid. If high concentration of bromine and high concentration of hydrobromic acid which leads to the formation of bromination in the double bond.

Bromination reaction starts with the homolytic cleavage of $\text{N-Br}$ bond in N-bromosuccinimide (NBS) which creates bromine radical to initiate the radical bromination reaction.

NBS bromine radical removes the allylic hydrogen which forms hydrogen bromide and allylic radical in the first propagation step, the allylic radical is stabilized by the double bond in ring. This allylic radical reaction with bromine molecule and forms allylic bromide in the second propagation step which are shown above.