Chapter 12 :  Reactions of Arenes. Electrophilic Aromatic Substitution

Limitations of Friedel-Crafts reactions

As we have seen previosuly there are two types of Friedel-Crafts reactions, alkylation and acylation:

Reaction type: Electrophilic Aromatic Substitutions

However there are certain limitations:

• The halide must be either  an alkyl halide or an acyl 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.
• The Lewis acid catalyst AlCl₃ often complexes to aryl amines making them very unreactive.

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