CIE A Level Chemistry

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7.2.3 Location of Halogenation on Arenes

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Arenes: Location of Halogenation

  • 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

 Hydrocarbons - Halogenation in the aromatic ring, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons - Carbon Halogen Bond Strength, downloadable AS & A Level Chemistry revision notes

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

 Hydrocarbons - Halogenation in the side-chain, downloadable AS & A Level Chemistry revision notes

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

 

Hydrocarbons - Complete halogenation in the side-chain, downloadable AS & A Level Chemistry revision notes

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

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