Chapter 19.6, Problem 9CC

Interpretation Introduction

Interpretation:

The inefficacy of the synthesis of the following compound –

                   

  By Friedel Crafts reaction has to be explained.

Concept Introduction:

Friedel Crafts reaction is an electrophilic substitution reaction. This reaction is useful to introduce alkyl, aryl or acyl group onto benzene ring. So this reaction is commonly known as Friedel Crafts acylation if acyl group is imparted on benzene ring or Friedel Crafts alkylation if alkyl group is imparted on benzene ring.

Benzene is highly stable that the sole presence of any alkyl or acyl halide is not enough to initiate the reaction. Lewis acids such as ${\text{AlCl}}_{\text{3,}}{\text{FeCl}}_{\text{3}}\text{,}{\text{BF}}_{\text{3}}\text{etc}$ is required to initiate the reaction. The Lewis acid catalyst assists in the formation of electrophile. Alkyl halide reacts with Lewis acid to generate alkyl ion as electrophile. The so formed alkyl ion must be highly stable in order to bond with benzene Carbon. Of all the carbocations primary, secondary and tertiary, tertiary carbocation is the most stable.

The type of carbocation initially formed depends upon the alkyl halide. But it rearranges itself to the most stable form. This phenomenon poses some drawbacks to Friedel Crafts reaction that some compounds can’t be prepared by this reaction.