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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

The bromine molecule is an electrophile; the π electrons of but‑1‑ene attack one Br atom, giving a bromonium ion (or a carbocation) on the double‑bonded carbons. Two intermediates are possible: a primary carbocation at C‑1 or a secondary carbocation at C‑2. The secondary carbocation is more stable because the two alkyl groups on C‑2 donate electron density and delocalise the positive charge. Therefore the secondary intermediate is formed preferentially. The nucleophile (Br⁻ or H₂O) then attacks mainly at C‑2, giving the major product (2‑bromobutane or 2‑bromobutanol). The less stable primary intermediate gives the minor product (1‑bromobutane or 1‑bromobutanol).

Examiner tips

  • Use the term ‘electrophile’ and ‘carbocation’ to show understanding of mechanism.
  • Explain relative stability of primary vs secondary carbocations to justify major/minor products.
  • Mention that nucleophile attacks the more stable intermediate.
  • Keep answer concise – 4 marks are enough for a short explanation.

Common mistakes

  • Confusing the bromonium ion with a simple carbocation; students often omit the ionisation step.
  • Failing to state that the secondary carbocation is more stable, or giving the wrong reason for stability.
  • Writing the products in the wrong order (major first).

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 in this question

major product

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