Location of Halogenation on Arenes (CIE A Level Chemistry)

• 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

A halogen carrier catalyst is used to generate the electrophile for the halogenation of alkylarenes 

• 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

The lack of reactivity in alkylarenes / aryl halides

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 an alkylarene side chain

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

Halogenation of an alkylarene side chain using excess halogen 

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