Extending Carbon Chains on Benzene

Benzene is very stable, but a new carbon−carbon bond can be formed to its ring by Friedel-Crafts reactions. Both are electrophilic substitutions: a hydrogen atom on the ring is replaced by a carbon group.Each reaction needs a halogen carrier, such as aluminium chloride, AlCl₃. The halogen carrier generates a strong electrophile that can attack the delocalised π-system of benzene. The reaction is carried out under anhydrous conditions.Friedel-Crafts alkylation uses a haloalkane to add an alkyl group:C₆H₆ + CH₃CH₂Cl → C₆H₅CH₂CH₃ + HClThe electrophile is a carbocation, formed as AlCl₃ removes the chlorine: CH₃CH₂Cl + AlCl₃ → CH₃CH₂⁺ + AlCl₄⁻. The product here is ethylbenzene.Friedel-Crafts acylation uses an acyl chloride to add an acyl group, R−C=O, making an aromatic ketone:C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HClThe product is phenylethanone. The electrophile is the acylium ion, CH₃CO⁺.In both reactions the halogen carrier is regenerated at the end: H⁺ + AlCl₄⁻ → AlCl₃ + HCl, so it acts as a catalyst.Worked example: to make propylbenzene, react benzene with 1-chloropropane and AlCl₃; to make phenylpropanone, use propanoyl chloride and AlCl₃.