Substituted Aromatic Carbon-carbon Bond Formation (OCR A Level Chemistry A): Revision Note

Alkylation & Acylation

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

Example of a Friedel-Crafts alkylation reaction

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 

Example of a Friedel-Crafts acylation reaction