Ch12: Friedel-Crafts acylation

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.
Alternatively, the acid anhydride, i.e. (RCO)₂O, can be used instead of the acyl halide.
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 stabilised 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

Summary of Limitations of Friedel-Crafts acylations:

Acylation can only be used to give ketones. This is because HCOCl decomposes to CO and HCl under the reaction conditions.
Deactivated benzenes are not reactive to Friedel-Crafts conditions, the benzene needs to be as or more reactive than a mono-halobenzene (see substituent effects)
The Lewis acid catalyst AlCl₃ often complexes to aryl amines making them very unreactive.
Amines and alcohols can give competing N or O acylations rather than the require ring acylation.

MECHANISM FOR THE FRIEDEL-CRAFTS ACYLATION OF BENZENE

Step 1:
The acyl halide reacts with the Lewis acid to form a complex.

Step 2:
Loss of the halide to the Lewis acid forms the electrophilic acylium ion.

Step 3:
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 4:
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.