Chapter 13, Problem 62DSP

Interpretation Introduction

Interpretation:

The isomer of bromodimethylbenzene that cannot undergo nucleophilic aromatic substitution by treatment with sodium amide in liquid ammonia is to be determined.

Concept introduction:

Aryl halides in the presence of a strong base, undergo substitution via elimination-addition mechanism.

In the first step, the base abstracts a proton attached to the ring carbon atom which is adjacent to the halogen. The proton and the halogen from adjacent carbon atoms are eliminated, and triple bond is created. The intermediate formed is benzyne.

The nucleophile then adds to any one of the triple bonded carbon atoms of the benzyne.

For the formation of the benzyne intermediate, it is necessary that one hydrogen atom be attached to the carbon atoms adjacent to the one that bears the halogen.