# Benzene reacts with bromine in the presence of an aluminium bromide catalyst to form bromobenzene and hydrogen bromide. This reaction is an example of electrophilic substitution. Explain the mechanism of this reaction, including how the electrophile is generated and why substitution rather than addition occurs.

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

> Benzene does not react with bromine water under normal conditions, but reacts readily with bromine in the presence of aluminium bromide as a halogen carrier catalyst.

## Mark scheme (5 marks)

1. Aluminium bromide acts as a Lewis acid / halogen carrier and accepts a lone pair from Br₂, polarising the Br–Br bond
2. This generates Br⁺ (or a species acting as Br⁺) as the electrophile
3. The electrophile is attracted to and attacks the delocalised electron ring / π system of benzene, forming an intermediate in which the ring loses its delocalisation
4. A proton (H⁺) is then lost from the ring / the intermediate loses H⁺ to [AlBr₄]⁻ to regenerate the aromatic ring
5. Substitution is favoured over addition because it restores / preserves the delocalised ring / aromatic stability, which would be lost if addition occurred

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/benzene-reacts-with-bromine-in-the-6db24909) · Published by Druglandscape Ltd.