Chapter 19.6, Problem 8CC

Interpretation Introduction

Interpretation:

The strategy involved in the synthesis of the following compounds has to be illustrated.

Concept Introduction:

• Friedel Crafts reaction is an electrophilic substitution reaction. This reaction is useful to introduce alkyl 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 mere 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 and generates a complex. The complex decomposes and forms alkyl ion which functions 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 order of the stability of carbocations can be represented as follows –

• $\text{Primary carbocation < Secondary carbocation < Tertiary carbocation}$

• 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. 

• Conversion of $\text{-CO}$ functional group to ${\text{-CH}}_2$ can be achieved by reaction known as Clemmenson reduction. Zinc-Mercury amalgam mixed with $\text{HCl}$ is the reducing agent in this reaction.