Explain why hex-1-ene reacts with hydrogen bromide to produce predominantly 2-bromohexane rather than 1-bromohexane, including reference to the stability of the intermediate species involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The addition of HBr to hex‑1‑ene is an electrophilic addition.
1. The H⁺ (or the δ⁺ hydrogen of HBr) is the electrophile and attacks the C=C double bond.
2. If H⁺ adds to C‑1, a secondary carbocation is formed at C‑2; if it adds to C‑2, a primary carbocation would form at C‑1.
3. The secondary carbocation is more stable than the primary one because the two alkyl groups on C‑2 donate electron density inductively and by hyperconjugation, delocalising the positive charge.
4. Br⁻ then attacks the more stable secondary carbocation at C‑2, giving 2‑bromohexane as the major product, in accordance with Markovnikov’s rule.
1. The H⁺ (or the δ⁺ hydrogen of HBr) is the electrophile and attacks the C=C double bond.
2. If H⁺ adds to C‑1, a secondary carbocation is formed at C‑2; if it adds to C‑2, a primary carbocation would form at C‑1.
3. The secondary carbocation is more stable than the primary one because the two alkyl groups on C‑2 donate electron density inductively and by hyperconjugation, delocalising the positive charge.
4. Br⁻ then attacks the more stable secondary carbocation at C‑2, giving 2‑bromohexane as the major product, in accordance with Markovnikov’s rule.
Examiner tips
- State the reaction type and the role of H⁺ first. Explain the two possible carbocations and compare their stability. Show how the more stable carbocation leads to the major product. Use the term ‘Markovnikov’s rule’ to link the reasoning to the product distribution.
Common mistakes
- Confusing the positions of the carbocation and the bromide attack. Forgetting to mention the relative stability of secondary vs primary carbocations. Using the wrong terminology (e.g., calling the intermediate a radical instead of a carbocation).
Mark scheme (4 marks)
- The reaction proceeds via electrophilic addition, with H⁺ (or the δ+ hydrogen of HBr) acting as the electrophile attacking the C=C double bond in the first step.
- Addition of H⁺ to C-1 generates a secondary carbocation at C-2, whereas addition to C-2 would generate a less stable primary carbocation at C-1.
- The secondary carbocation is more stable than the primary carbocation because alkyl groups donate electron density inductively / hyperconjugation, delocalising the positive charge more effectively.
- Br⁻ (the nucleophile) then attacks the more stable secondary carbocation at C-2, giving 2-bromohexane as the major product, in accordance with Markovnikov's rule.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →