# Explain why cyclohexene reacts with bromine to produce only one dibromide product, whereas pent-1-ene reacts with bromine to produce two stereoisomeric dibromide products.

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

## Mark scheme (4 marks)

1. Electrophilic addition of bromine proceeds via a cyclic bromonium ion intermediate, in which the bromine bridges the two carbons of the double bond, blocking one face of the molecule.
2. The bromide ion nucleophile attacks the bromonium ion from the opposite face (anti addition), so both new C–Br bonds form with inversion, giving only anti addition products.
3. For cyclohexene, anti addition of two bromide ions to the ring gives only the trans-1,2-dibromocyclohexane, and because the ring constrains the molecule symmetrically, only one (racemic) product results.
4. For pent-1-ene, anti addition to the bromonium ion at C-1 and C-2 gives a product with two chiral centres; the bromide can attack either C-1 or C-2 of the unsymmetrical bromonium ion, and the two possible anti addition pathways yield two different stereoisomers (enantiomers or a pair of products), hence two stereoisomeric dibromides are obtained.

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-cyclohexene-reacts-with-bromine-1200cefb) · Published by Druglandscape Ltd.