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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cyclohexene decolourises bromine water immediately at room temperature. Benzene does not decolourise bromine water under the same conditions.
Model answer (5 marks)
Benzene contains a delocalised system of π electrons that is spread over all six carbon atoms, forming an electron cloud above and below the ring.
This delocalisation gives benzene extra stability – a stabilisation energy that is greater than that of a simple alkene.
Because of this extra stability, benzene resists reactions that would break or disrupt the delocalised electron system.
Cyclohexene, in contrast, has a localised C=C double bond; the π electrons are confined to that bond and can be added to by bromine, giving an addition product.
Benzene would require much higher energy (e.g. a catalyst or harsh conditions) to undergo such addition, so it does not decolourise bromine water at room temperature.
This delocalisation gives benzene extra stability – a stabilisation energy that is greater than that of a simple alkene.
Because of this extra stability, benzene resists reactions that would break or disrupt the delocalised electron system.
Cyclohexene, in contrast, has a localised C=C double bond; the π electrons are confined to that bond and can be added to by bromine, giving an addition product.
Benzene would require much higher energy (e.g. a catalyst or harsh conditions) to undergo such addition, so it does not decolourise bromine water at room temperature.
Examiner tips
- Use the term "delocalised π electrons" and "extra stability" – these are key phrases. Explain that the stability prevents addition reactions. Mention that cyclohexene has a conventional double bond that reacts by addition. Keep the answer to 5 points, one per sentence.
Common mistakes
- Saying benzene reacts by substitution instead of explaining delocalisation. Forgetting to mention the extra stability or the need for harsh conditions. Using vague terms like "reacts slowly" without linking to electron delocalisation.
Mark scheme (5 marks)
- Benzene has a delocalised system of electrons (delocalised pi electrons / electron cloud) spread over all six carbon atoms in the ring
- This delocalisation gives benzene extra stability (greater than expected stability / stabilisation energy)
- Because of this stability, benzene resists reactions that would disrupt or break into the delocalised electron system
- Cyclohexene has a localised (conventional) carbon–carbon double bond which reacts readily with bromine by addition
- 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
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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