Explain why but-1-ene reacts with bromine water to give predominantly 2-bromobutanol rather than 1-bromobutanol as the major organic product.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Bromine water (Br₂(aq)) reacts with but-1-ene (CH₂=CHCH₂CH₃) via an electrophilic addition mechanism. The reaction proceeds through a bromonium ion intermediate rather than a simple carbocation.
Model answer (4 marks)
Br₂ approaches the π bond and the more electron‑rich bromine acts as an electrophile, forming a cyclic bromonium ion intermediate across the double bond.
The bromonium ion is unsymmetrical because the C‑2 atom can stabilise the developing positive charge better than C‑1; consequently the C–Br bond at C‑2 is weaker and the C‑2 centre carries a greater partial positive charge.
Water, as the nucleophile, attacks preferentially at C‑2, the more positively charged carbon, giving a 2‑bromobutyl oxonium ion.
Deprotonation of this intermediate yields 2‑bromobutanol; the attack occurs anti to the leaving Br, giving the anti‑addition product.
The bromonium ion is unsymmetrical because the C‑2 atom can stabilise the developing positive charge better than C‑1; consequently the C–Br bond at C‑2 is weaker and the C‑2 centre carries a greater partial positive charge.
Water, as the nucleophile, attacks preferentially at C‑2, the more positively charged carbon, giving a 2‑bromobutyl oxonium ion.
Deprotonation of this intermediate yields 2‑bromobutanol; the attack occurs anti to the leaving Br, giving the anti‑addition product.
Examiner tips
- Use the word ‘electrophile’ and ‘nucleophile’ to show understanding of the mechanism.
- Show that the bromonium ion is unsymmetrical and explain why C‑2 is more positive.
- Mention anti‑addition to justify the stereochemistry.
- Keep the answer concise – 4 points, 1 sentence per point.
Common mistakes
- Confusing the roles of Br₂ and water (calling Br₂ the nucleophile).
- Failing to explain why the bromonium ion is unsymmetrical and why C‑2 is more positive.
- Omitting the anti‑addition aspect of the attack.
Mark scheme (4 marks)
- Br₂ approaches the π bond and the more electron-rich bromine acts as an electrophile, forming a cyclic bromonium ion intermediate across the double bond
- The bromonium ion is unsymmetrical because C-2 can better stabilise partial positive charge than C-1, so the C–Br bond at C-2 is weaker / longer
- Water (the nucleophile) attacks preferentially at C-2 because it carries the greater partial positive charge
- Attack of water at C-2 followed by loss of a proton gives 2-bromobutanol; attack is anti (from the opposite face to Br) giving the anti-addition product
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →