7.6.4 The Friedel-Crafts Alkylation

In the presence of Lewis acid catalysts such as aluminum chloride (AlCl₃) or ferric chloride (FeCl₃), alkyl halides react with benzene to give alkyl benzenes (Scheme 6). This reaction is called the Friedel-Crafts alkylation.

Scheme 6

This alkylation is an electrophilic aromatic substitution reaction where the tert-butyl cation acts as the electrophile. The tert-butyl cation is formed by the reaction of tert-butyl chloride with the catalyst, aluminum chloride. The tert-butyl cation reacts with benzene to form a sigma complex. Loss of a proton gives the product. The aluminum chloride catalyst is regenerated in the final step.

7.6.5 The Friedel-Crafts Acylation

In the presence of aluminum chloride, an acyl chloride reacts with benzene to give acyl benzene (Scheme 7). The Friedel-Crafts acylation is analogous to the Friedel-Crafts alkylation, except that the reagent is acyl chloride instead of an alkyl halide and the product is acyl benzene instead of alkyl benzene .

Scheme 7

In the first step a resonance-stabilized acylium ion formed which reacts with benzene via an electrophilic aromatic substitution reaction to form an acyl benzene. The carbonyl group in the product has nonbonding electrons that can form a complex with the Lewis acid (AlCl₃). Addition of water hydrolyzes this complex, giving the free acyl benzene (Scheme 8). Friedel-Crafts reactions do not occur on strongly deactivated rings, so the acylation stops after one substitution.

Scheme 8

Friedel-Crafts acylations can also be carried out using carboxylic acid anhydrides. For example, benzene reacts with acetic anhydride in the presence of Lewis acid to give acetophenone (Scheme 9). Excess of benzene is used in this reaction to get good yield.

Scheme 9