Benzene, C₆H₆, is an aromatic hydrocarbon. A student claims that benzene should react readily with bromine water because it contains carbon-carbon double bonds. Evaluate this claim, referring to the structure and stability of benzene in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Benzene does not react readily with bromine water – the solution does not decolourise.
The claim is wrong because benzene does not contain isolated C=C double bonds. The π‑electrons are delocalised over the whole ring, giving all C–C bonds the same length, intermediate between a single and a double bond.
Delocalisation stabilises the ring (aromatic stabilisation energy ≈ 36 kJ mol⁻¹). Breaking this delocalised system would require a large amount of energy, so benzene favours substitution reactions that preserve the aromatic π‑system rather than addition reactions that would destroy it.
Hence bromine water does not react with benzene, and the stability of the aromatic ring explains this behaviour.
The claim is wrong because benzene does not contain isolated C=C double bonds. The π‑electrons are delocalised over the whole ring, giving all C–C bonds the same length, intermediate between a single and a double bond.
Delocalisation stabilises the ring (aromatic stabilisation energy ≈ 36 kJ mol⁻¹). Breaking this delocalised system would require a large amount of energy, so benzene favours substitution reactions that preserve the aromatic π‑system rather than addition reactions that would destroy it.
Hence bromine water does not react with benzene, and the stability of the aromatic ring explains this behaviour.
Examiner tips
- State that benzene is unreactive to bromine water; explain delocalisation and equal bond lengths; mention aromatic stabilisation energy; note substitution preserves aromaticity; keep answer concise and use exam terminology.
Common mistakes
- Claiming benzene reacts with bromine water; confusing benzene with alkenes; ignoring the need to preserve aromaticity in reactions.
Mark scheme (5 marks)
- The student's claim is incorrect / benzene does not readily react with bromine water (does not decolourise bromine water)
- Benzene does not contain localised carbon-carbon double bonds; the electrons are delocalised / spread around the ring
- All carbon-carbon bonds in benzene are equal / intermediate in length between a single and a double bond
- The delocalisation makes benzene more stable than expected / gives benzene extra stability (delocalisation / aromatic stabilisation energy)
- Because disrupting the delocalised system requires a lot of energy, benzene undergoes substitution rather than addition (to preserve the stable aromatic ring)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
More Aromaticity questions
- Benzene is described as an aromatic compound. Explain the bonding in benzene and…
- A student studies two cyclic compounds: cyclohexane and benzene. The student not…
- A chemist compares the stability of benzene with that of a theoretical compound …
- Cyclohexene and benzene are both cyclic hydrocarbons with the molecular formula …