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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Aluminium bromide is a Lewis acid that accepts a lone pair from Br₂, polarising the Br–Br bond and generating a Br⁺ (or a Br⁺‑like species) as the electrophile.
The Br⁺ is attracted to the delocalised π system of benzene and attacks the ring, forming a σ‑complex (arenium ion) in which the aromaticity of the ring is temporarily lost.
A proton is then removed from the σ‑complex, usually by the [AlBr₄]⁻ anion, regenerating the aromatic ring and giving bromobenzene.
Substitution is favoured over addition because the addition product would destroy the aromatic system, whereas substitution restores the delocalised π electrons and preserves aromatic stability.
The Br⁺ is attracted to the delocalised π system of benzene and attacks the ring, forming a σ‑complex (arenium ion) in which the aromaticity of the ring is temporarily lost.
A proton is then removed from the σ‑complex, usually by the [AlBr₄]⁻ anion, regenerating the aromatic ring and giving bromobenzene.
Substitution is favoured over addition because the addition product would destroy the aromatic system, whereas substitution restores the delocalised π electrons and preserves aromatic stability.
Examiner tips
- Show the Lewis acid–base step first, then the attack on the ring, then deprotonation; use the term σ‑complex or arenium ion.
- Mention that AlBr₃ acts as a halogen carrier, not a catalyst that changes the mechanism.
Mark scheme (5 marks)
- Aluminium bromide acts as a Lewis acid / halogen carrier and accepts a lone pair from Br₂, polarising the Br–Br bond
- This generates Br⁺ (or a species acting as Br⁺) as the electrophile
- 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
- A proton (H⁺) is then lost from the ring / the intermediate loses H⁺ to [AlBr₄]⁻ to regenerate the aromatic ring
- Substitution is favoured over addition because it restores / preserves the delocalised ring / aromatic stability, which would be lost if addition occurred
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Aromatic chemistry questions
- Benzene undergoes nitration when heated with a mixture of concentrated nitric ac…
- Benzene is more stable than expected based on its molecular formula. This extra …
- Benzene is an aromatic compound with the molecular formula C₆H₆. Explain why the…
- Chlorobenzene can be formed by the reaction of benzene with chlorine in the pres…