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Chapter 12 : 
Reactions of Arenes. Electrophilic Aromatic Substitution
Ch 12 contents

Friedel-Crafts Acylation of Benzene

acylation of arenes

Reaction type: Electrophilic Aromatic Substitution

Summary

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.

Friedel-Crafts acylation of benzene

 


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© Dr. Ian Hunt, Department of Chemistry, University of Calgary