# Explain why cyclohexene decolourises bromine water rapidly at room temperature, whereas benzene does not react with bromine water under the same conditions.

> IB DP Chemistry Standard Level (2023 syllabus) — R3.3 Electron sharing reactions · Explain · 4 marks

## Mark scheme (4 marks)

1. Cyclohexene contains a C=C (pi bond / double bond) that acts as a region of high electron density, attracting electrophiles such as Br₂.
2. Cyclohexene undergoes electrophilic addition with bromine, breaking the pi bond and adding Br across the double bond, which explains the rapid decolourisation.
3. 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.
4. Addition to benzene would destroy the delocalised system and lose the aromatic stabilisation energy, so electrophilic addition does not occur under these mild conditions.

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