Friedel–Crafts Acylation (AQA A Level Chemistry): Revision Note

Friedel–Crafts Acylation

• Friedel-Crafts reactions are 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

• A Friedel-Crafts reaction can be used to substitute a hydrogen atom in the benzene ring for an acyl group

  • 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

• The general reaction is:

benzene + acyl chloride $\to$ phenylethanone + HCl

C₆H₆ + CH₃COCl $\to$ C₆H₅COCH₃ + HCl

• Like other electrophilic substitution reactions, the Friedel-Crafts reaction consists of three steps:

  • Generating the electrophile

  • Electrophilic attack on the benzene ring

  • Regenerating aromaticity of the benzene ring

• An example of an acylation reaction is the reaction of methylbenzene with propanoyl chloride to form an acylbenzene

  • Note that the acyl group is in the 4 position due to the -CH₃ group on the benzene

Mechanism of acylation

Step 1 – Formation of the electrophile

• AlCl₃ reacts with the acyl chloride to produce the acylium ion

Step 2 – Attack by the benzene ring

• The delocalised π electrons attack the acylium ion, forming a non-aromatic positive intermediate

• Draw a curly arrow from the benzene ring to the carbon of the acylium ion

• One double bond disappears temporarily

Step 3 – Regeneration of aromaticity

• AlCl₄^- removes the hydrogen ion from the intermediate

• Aromaticity is restored

• HCl is formed

• AlCl₃ is regenerated

Related topics

• Structure of Benzene

• Nitration of Benzene

• Acylation

• Organic Synthesis