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

Friedel-Crafts Alkylation of Benzene

alkylation of arenes

Reaction type: Electrophilic Aromatic Substitution


alkylation with rearrangement1,2-hydride shift during a Friedel-Crafts reaction
Summary of Limitations of Friedel-Crafts alkylations:
  • The halide must be either  an alkyl halide. Vinyl or aryl halides do not react (their intermediate carbocations are too unstable).
  • Alkylation reactions are prone to carbocation rearrangements.
  • 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)
  • Over alkylation can be a problem since the product is more reactive than the starting material. This can usually be controlled with an excess of the benzene.
  • The Lewis acid catalyst AlCl3 often complexes to aryl amines making them very unreactive.



Step 1:
The alkyl halide reacts with the Lewis acid to form a a more electrophilic C, a carbocation


Step 2:
Loss of the halide to the Lewis acid forms the electrophilic alkyl carbocation.

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 alkyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.

Friedel-Crafts alkylation of benzene


previous page
next page
organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary