# Explain why but-1-ene reacts with bromine water to give predominantly 2-bromobutanol rather than 1-bromobutanol as the major organic product.

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

> 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.

## Mark scheme (4 marks)

1. 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
2. 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
3. Water (the nucleophile) attacks preferentially at C-2 because it carries the greater partial positive charge
4. 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

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