• Arenes will undergo substitution reactions?with halogens to form aryl halides
  • This reaction is also called a?halogenation?reaction
• Depending on the reaction conditions, halogenation can occur:
  • In the aromatic ring
  • In the side chain

Halogenation in the aromatic ring

• Halogenation of alkylarenes?in the aromatic ring?will occur when a halogen and anhydrous halogen carrier catalyst (such as AlBr₃?or AlCl₃) is used

Halogenation of alkylarenes in the aromatic ring

• Aryl halides are?less reactive?than?halogenoalkanes?as the?carbon-halogen?bond in aryl halides is?stronger
• This is due to the partial overlap of the lone pairs on the halogen atom with the π system in the benzene ring
• The carbon-halogen bond, therefore, has a?partial double bond character

Aryl halides are unreactive due to the partial double bond character of the carbon-halogen bond

Halogenation in the side chain

• Halogenation of alkylarenes?in the side chain?will occur when the halogen is passed into?boiling alkylarene in the presence of?ultraviolet?(UV)?light
  • This is a?free-radical substitution?reaction

Halogenation of alkylarenes in the side chain is an example of a free-radical substitution reaction

• If?excess halogen is used, all hydrogen atoms on the alkyl side-chain will be substituted by the halogen atoms

In excess halogen, all hydrogen atoms on the alkyl side-chain will be replaced

• Note?that no substitution into the benzene ring occurs under these conditions