# Cyclohexene and benzene are both cyclic hydrocarbons with the molecular formula suggesting multiple carbon–carbon bonds. A student tests both compounds with bromine water. Cyclohexene decolourises the bromine water immediately, but benzene does not decolourise it under the same conditions. Explain, in terms of the structure and bonding of benzene, why benzene does not react with bromine water in the same way as cyclohexene.

> WJEC A-Level Chemistry (Wales) — 4.2 Aromaticity · Explain · 5 marks

> Cyclohexene decolourises bromine water immediately at room temperature. Benzene does not decolourise bromine water under the same conditions.

## Mark scheme (5 marks)

1. Benzene has a delocalised system of electrons (delocalised pi electrons / electron cloud) spread over all six carbon atoms in the ring
2. This delocalisation gives benzene extra stability (greater than expected stability / stabilisation energy)
3. Because of this stability, benzene resists reactions that would disrupt or break into the delocalised electron system
4. Cyclohexene has a localised (conventional) carbon–carbon double bond which reacts readily with bromine by addition
5. Benzene would require much higher energy (e.g. a catalyst / harsh conditions) to react, which is why it does not decolourise bromine water at room temperature

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