# 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

> 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

## 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

- [electrophile](https://www.gradenine.co.uk/glossary/electrophile)

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