6.7.2 Substituted Aromatic Carbon-carbon Bond Formation

Friedel-Crafts Reactions

• Friedel-Crafts reactions are also electrophilic substitution reactions
• Due to the aromatic stabilisation in arenes, they are often unreactive
• To use arenes as starting materials for the synthesis of other organic compounds, their structure, therefore, needs to be changed to turn them into more reactive compounds
• Friedel-Crafts reactions can be used to substitute a hydrogen atom in the benzene ring for an alkyl group (Friedel-Crafts alkylation) or an acyl group (Friedel-Crafts acylation)
• Like any other electrophilic substitution reaction, the Friedel-Crafts reactions consist of three steps:
  • Generating the electrophile
  • Electrophilic attack on the benzene ring
  • Regenerating aromaticity of the benzene ring

Friedel-Crafts Alkylation 

• In the Friedel-Crafts alkylation, an alkyl group is substituted into the benzene ring
• The benzene ring is reacted with a chloroalkane (e.g. chloroethane) in the presence of an AlCl₃ catalyst 
• An example of alkylation is the reaction of benzene with chloroprpane to form propylbenzene
• The Friedel-Crafts alkylation reaction is a method of generating alkylbenzenes by using alkyl halides as reactants

Friedel-Crafts Acylation

• In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring
  • An acyl group is an alkyl group containing a carbonyl, C=O group

• The benzene ring is reacted with an acyl chloride in the presence of an AlCl₃ catalyst
• An example of an acylation reaction is the reaction of benzene with propanoyl chloride to form phenylpropanone 
  • The Friedel-Crafts alcylation reaction is a method of generating ketones 

Examples of Friedel-Crafts alkylation and acylation reactions