Explain why cyclohexene decolourises bromine water rapidly at room temperature, whereas benzene does not react with bromine water under the same conditions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Cyclohexene contains a C=C double bond, a region of high electron density that attracts electrophiles such as Br₂. The bromine adds across the double bond in an electrophilic addition, breaking the π bond and producing 1,2‑dibromocyclohexane, which explains the rapid loss of the orange colour of bromine water.
Benzene, however, has its π electrons delocalised over the whole ring, giving a highly stabilised aromatic system. Electrophilic addition would destroy this delocalisation and cost the aromatic stabilisation energy, so under mild conditions the reaction is unfavourable and benzene does not decolourise bromine water.
Benzene, however, has its π electrons delocalised over the whole ring, giving a highly stabilised aromatic system. Electrophilic addition would destroy this delocalisation and cost the aromatic stabilisation energy, so under mild conditions the reaction is unfavourable and benzene does not decolourise bromine water.
Examiner tips
- Mention the π bond in cyclohexene and its role as an electrophile site. Explain that addition breaks the π bond, giving a clear reason for decolourisation. State that benzene’s delocalised π system is stabilised. Note that addition would destroy aromaticity and is therefore unfavourable.
Common mistakes
- Confusing the reaction with a substitution mechanism. Forgetting to mention the loss of aromatic stabilisation energy for benzene. Using vague terms like "reacts" without specifying electrophilic addition.
Mark scheme (4 marks)
- Cyclohexene contains a C=C (pi bond / double bond) that acts as a region of high electron density, attracting electrophiles such as Br₂.
- Cyclohexene undergoes electrophilic addition with bromine, breaking the pi bond and adding Br across the double bond, which explains the rapid decolourisation.
- Benzene has delocalised electrons spread over the ring in a stable pi system, making it less susceptible to electrophilic attack than an isolated C=C.
- Addition to benzene would destroy the delocalised system and lose the aromatic stabilisation energy, so electrophilic addition does not occur under these mild conditions.
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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