Chapter 12 :
Reactions of Arenes. Electrophilic Aromatic Substitution
Friedel-Crafts Acylation of Benzene
Reaction type: Electrophilic Aromatic Substitution
- Overall transformation : Ar-H to Ar-COR(a
- 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
- 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
- Friedel-Crafts reactions are limited to arenes as or more reactive than
- Other sources of acylium can also be used such as acid anhydrides with
|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
The acyl halide reacts with the Lewis acid to form a complex.
Loss of the halide to the Lewis acid forms the electrophilic acylium ion.
The p electrons of the aromatic C=C act as a nucleophile, attacking
the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl
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.