Chapter 12 :  Reactions of Arenes. Electrophilic Aromatic Substitution

Friedel-Crafts Acylation of Benzene

Reaction type: Electrophilic Aromatic Substitution

Summary

• Overall transformation :  Ar-H to Ar-COR (a ketone)
• Named after Friedel and Crafts who discovered the reaction.
• Reagent : normally the acyl halide (e.g. usually RCOCl) with aluminum trichloride, AlCl₃, a Lewis acid catalyst
• The AlCl₃ enhances the electrophilicity of the acyl halide by complexing with the halide
• Electrophilic species : the acyl cation or acylium ion (i.e. RCO ⁺ ) formed by the "removal" of the halide by the Lewis acid catalyst
• The acylium ion is stabilized by resonance as shown below. This extra stability prevents the problems associated with the rearrangement of simple carbocations:

• The reduction of acylation products can be used to give the equivalent of alkylation but avoids the problems of rearrangement  (more details)
• Friedel-Crafts reactions are limited to arenes as or more reactive than mono-halobenzenes
• Other sources of acylium can also be used such as acid anhydrides with AlCl₃

MECHANISM FOR THE FRIEDEL-CRAFTS ACYLATION OF BENZENE
Step 1: The acyl halide reacts with the Lewis acid to form a a more electrophilic C, an acylium ion
Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3: Removal of the proton from the sp3 C bearing the acyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.