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A-Level Chemistry May/June 2025 Q3(a)(iii): Explain why the major product of this reaction is 2-bromopropane rather than 1-bromopro…
A-Level Chemistry · Paper 9701/22 · May/June 2025 · Question 3(a)(iii) · [2 marks]
Explain why the major product of this reaction is 2-bromopropane rather than 1-bromopropane.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The reaction proceeds via a carbocation intermediate. The formation of 2-bromopropane occurs via a secondary carbocation, while 1-bromopropane is formed via a primary carbocation.
The secondary carbocation is more stable than the primary carbocation. This is because the positive charge on the central carbon atom is stabilised by the positive inductive effect of the two adjacent methyl groups, which donate electron density. The primary carbocation is only stabilised by the inductive effect of one ethyl group. As the secondary carbocation intermediate is more stable, it is formed in preference, leading to 2-bromopropane as the major product.
How the marks are awarded
- M1 — Stating that the major product (2-bromopropane) is formed via the more stable carbocation intermediate.
- M2 — Explaining that the greater stability is due to the positive inductive effect of the two alkyl (methyl) groups attached to the positively charged carbon, which is a greater effect than that from the single alkyl (ethyl) group in the primary carbocation.
Common mistakes
- Simply stating 'Markovnikov's rule' without providing the required explanation of carbocation stability.
- Failing to explain why the secondary carbocation is more stable, e.g., just stating 'it is more stable' without mentioning the inductive effect.
- Incorrectly describing the inductive effect, for example stating that alkyl groups withdraw electron density.
- Confusing the types of carbocation, e.g., referring to a primary carbocation as secondary.
Examiner tip: For reactions involving intermediates, always justify the formation of the major product by comparing the relative stabilities of the possible intermediates and explaining the electronic effects responsible.
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