Chapter 17: Aldehydes and Ketones. Nucleophilic Additionto C=O

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

Reaction type: Electrophilic Aromatic Substitution

Summary.

• Overall transformation :  Ar-H toAr-COR(anaryl ketone).
• 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 bycomplexing with the halide.
• Electrophilic species : the acyl cation or acylium ion (i.e.RCO⁺ ) formed by the "removal" of the halide by the Lewisacid catalyst.
• The acylium ion is stabilised by resonance as shown below. This extra stabilitypreventsthe problems associated with the rearrangement of simple carbocations:

• The reduction of acylation products can be used to give the equivalentof 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 withAlCl₃

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

© Dr. Ian Hunt, Department of Chemistry