# Explain why but-1-ene reacts with bromine water to produce a major product and a minor product, rather than a single product.

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

> But-1-ene (CH₂=CHCH₂CH₃) reacts with bromine water (Br₂/H₂O) via electrophilic addition. Two constitutional isomers are formed: one in greater quantity than the other.

## Mark scheme (4 marks)

1. The bromine molecule acts as an electrophile and the π electrons of but-1-ene attack Br₂, forming a carbocation intermediate (or bromonium ion) at one of the two carbon atoms of the double bond.
2. The carbocation can form at C-1 (primary carbocation) or at C-2 (secondary carbocation), giving two possible intermediates.
3. The secondary carbocation (positive charge on C-2) is more stable than the primary carbocation (positive charge on C-1) because the alkyl groups attached to C-2 donate electron density, dispersing the positive charge.
4. The more stable secondary carbocation forms preferentially, so the nucleophile (Br⁻ or H₂O) attacks mainly at C-2, producing the major product (2-bromobutane or 2-bromobutanol) in greater amount than the minor product (1-bromobutane or 1-bromobutanol).

## Key terms

- [major product](https://www.gradenine.co.uk/glossary/major-product)

## Related

- [Revision notes for IB DP Chemistry Standard 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-but-1-ene-reacts-with-bromine-de6618e4) · Published by Druglandscape Ltd.