CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.3.2 Location of Halogenation on Arenes

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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