Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution

Friedel-Crafts Acylation of Benzene
(review of Chapter 12)

Reaction type: Electrophilic Aromatic Substitution (of the aromatic), or
                         Nucleophilic Acyl Substitution (of the acyl halide)

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.
• 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
• Note how the reaction can still be reviewed as a Nucleophilic Acyl Substitution of the acyl halide since overall we have a nucleophile (here the p bond of an aromatic ring) replaces the leaving group (chloride) at the electrophilic C=O.