Chlorobenzene can be formed by the reaction of benzene with chlorine in the presence of an aluminium chloride catalyst. Explain why benzene undergoes substitution rather than addition in this reaction, and state what type of reagent acts as the electrophile in this reaction.

Edexcel A-Level Chemistry (9CH0) — 18.1 Aromatic chemistry · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Benzene reacts with chlorine in the presence of anhydrous aluminium chloride to form chlorobenzene and hydrogen chloride. The overall equation is: C₆H₆ + Cl₂ → C₆H₅Cl + HCl

Model answer (5 marks)

Benzene possesses a ring of delocalised π electrons that gives it extra stability (delocalisation energy). In an addition reaction the π system would be broken, destroying aromaticity and the stabilisation energy. Substitution, however, replaces one H atom with a substituent while the π system remains intact, so aromaticity is preserved and the product is more stable. AlCl₃ is a Lewis acid that reacts with Cl₂ to form a polarised Cl–Cl–AlCl₄⁻ complex, generating the electrophile Cl⁺ (or a Clδ⁺ species).

Examiner tips

  • Use the term ‘delocalised π electrons’ and ‘aromaticity’ to show understanding of stability. Mention that addition would break the ring, substitution preserves it. Cite AlCl₃ as a Lewis acid and state the electrophile as Cl⁺. Keep the answer concise and to the point.

Common mistakes

  • Forgetting to mention that aromaticity is preserved in substitution. Confusing the role of AlCl₃ – describing it as a nucleophile instead of a Lewis acid. Stating the electrophile as Cl₂ instead of Cl⁺ or a polarised complex.

Mark scheme (5 marks)

  1. Benzene has a delocalised system of electrons / a ring of delocalised π electrons that gives it extra stability (delocalisation energy / stabilisation energy)
  2. Addition would destroy/disrupt the delocalised ring / break the aromatic system
  3. Substitution preserves the delocalised ring / retains aromaticity, so the product is more stable
  4. AlCl₃ acts as a Lewis acid / halogen carrier and generates the electrophile by accepting a lone pair / by reacting with Cl₂ to form Cl⁺ or the Cl–AlCl₃⁻ ion pair
  5. The electrophile is Cl⁺ (or the polarised Cl–Cl–AlCl₄⁻ complex / Clδ+)

Key terms in this question

electrophile

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