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

Review of Limitations of Friedel-Crafts reactions

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

alkylation  alkylation
acylationacylation

Reaction type: Electrophilic Aromatic Substitutions

However there are certain limitations:

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.

 

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.

QUESTION : Why aren't acylation reactions as prone to over acylation ? ANSWER


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