Name and draw the mechanism by which benzene reacts with ethanoyl chloride in the presence of a catalyst. Also explain why benzene undergoes a substitution reaction, rather than an addition reaction.

Electrophilic substitution. (drawn) AlCl₃ + CH₃COCl --> AlCl₄^- + CH₃CO⁺ (drawn) Then the electrophile CH₃CO⁺ attracts electrons from benzene's delocalised ring to form an intermediate, that the AlCl₄^- reacts with to remove the hydrogen, producing HCl, AlCl₃, and phenylethanone. Benzene undergoes substitution rather than addition because this maintains the stable delocalised ring in benzene.