CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.3.1 Halogen Compounds

Difference in Reactivity of Halogenoalkanes & Halogenoarenes

  • Halogenoarenes are very unreactive compared to halogenoalkanes
  • The difference in reactivity between the two compounds is because of the carbon-halogen bond strengths


  • The halogenoalkane chloroethane can take part in nucleophilic substitution reactions
  • A nucleophile, such as a hydroxide (OH) ion, will attack the slightly positive carbon atom
  • A covalent bond is formed between that carbon atom and the nucleophile which causes the carbon-halogen bond to break
  • Overall, the halogen is replaced by the nucleophile

Halogen Compounds - Nucleophilic Substitution of Halogenoalkane, downloadable AS & A Level Chemistry revision notes

Halogenoalkanes readily undergo nucleophilic substitution reactions


  • Halogenoarenes, such as chlorobenzene, do not readily undergo nucleophilic substitution reactions
    • Only under extremely harsh conditions, such as temperatures of 200 oC and a pressure of 200 atmospheres, will the chlorine in chlorobenzene get replaced by a nucleophile such as a hydroxide (OH) ion
  • This is because the carbon-chlorine bond is very strong and cannot be easily broken
    • One of the lone pairs of electrons on the chlorine will interact with the π system of the ring
    • This causes the carbon-chlorine bond to have a partial double-bond character, which strengthens the bond

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

The carbon-chlorine bond is very strong, as it has partial double-bond character

  • The unreactivity of halogenoarenes can therefore be explained by the delocalisation of a lone pair on the halogen over the benzene
  • This causes additional stabilisation of the system and strengthens the carbon-halogen bond, which affects the reactions that halogenoarenes will undergo
  • It gets harder to break the carbon-halogen bond in halogenoarenes, which decreases reactivity

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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