Free Radical Substitution of Arenes (Oxford AQA International A Level Chemistry)

Free Radical Substitution of Arenes

Free radical reactions

• When exposed to ultraviolet (UV) light chlorine will react with benzene in an addition reaction

• Homolytic fission occurs to the chlorine molecule

  • Cl-Cl 2Cl^•

• The product formed is 1,2,3,4,5,6-hexachlorocyclohexane

Methylbenzene

• The methyl group in methylbenzene will undergo a substitution reaction

• The benzene group will not undergo an addition reaction

• Therefore, methylbenzene will behave as an alkane

  • Each separate substitution reaction forms a molecule of HCl

C₆H₅CH₃ + Cl₂ C₆H₅CH₂Cl + HCl

Reactions of halogen-substituted arenes

• Alkylbenzenes such as methylbenzene can have halogen substitutions on:

  • The alkyl group

    • The Cl group can undergo nucleophilic substitution when heated with alkaline solution containing OH^- ions

    • This forms phenylchloromethane

  • The ring

    • This can only be substituted under very extreme conditions due to high electron density on the benzene ring

    • This forms 1-chloromethylbenzene

Phenylchloromethane and 1-chloromethylbenzene