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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Electrophilic addition of Br₂ to an alkene proceeds through a cyclic bromonium ion.
1. The Br₂ adds to the double bond, forming a three‑membered bromonium ion that bridges the two carbon atoms.
2. The ring of the bromonium ion blocks one face of the molecule; only the opposite face is accessible.
3. The Br⁻ nucleophile attacks the bromonium ion from the opposite side (anti addition), giving inversion at both carbons.
4. In cyclohexene the bromonium ion is symmetrical; anti attack gives only the trans‑1,2‑dibromocyclohexane (a racemic mixture). In pent‑1‑ene the bromonium ion is unsymmetrical, so Br⁻ can attack either C‑1 or C‑2, producing two different anti‑addition pathways and two stereoisomeric dibromides.
1. The Br₂ adds to the double bond, forming a three‑membered bromonium ion that bridges the two carbon atoms.
2. The ring of the bromonium ion blocks one face of the molecule; only the opposite face is accessible.
3. The Br⁻ nucleophile attacks the bromonium ion from the opposite side (anti addition), giving inversion at both carbons.
4. In cyclohexene the bromonium ion is symmetrical; anti attack gives only the trans‑1,2‑dibromocyclohexane (a racemic mixture). In pent‑1‑ene the bromonium ion is unsymmetrical, so Br⁻ can attack either C‑1 or C‑2, producing two different anti‑addition pathways and two stereoisomeric dibromides.
Examiner tips
- Use the word ‘anti addition’ to show you know the stereochemical outcome.
- Mention the bromonium ion intermediate – examiners expect this step. Explain why the ring symmetry limits cyclohexene to one product, while the open chain gives two.
- Show the two possible attack sites for pent‑1‑ene to justify two stereoisomers.
Common mistakes
- Failing to mention the bromonium ion or anti addition. Assuming both alkenes give the same number of products. Not recognising the symmetry of cyclohexene that limits the product to one stereoisomer.
Mark scheme (4 marks)
- 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.
- 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.
- 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.
- 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
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →