Explain why propene reacts with concentrated sulfuric acid to form propan-2-yl hydrogensulfate as the major product rather than propan-1-yl hydrogensulfate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The π electrons of propene act as a nucleophile and attack the electrophilic hydrogen of H₂SO₄, forming a carbocation intermediate.
Protonation at C‑1 gives a secondary carbocation at C‑2, whereas protonation at C‑2 gives a primary carbocation at C‑1.
The secondary carbocation is more stable than the primary carbocation because the additional alkyl group donates electron density inductively, dispersing the positive charge.
The hydrogensulfate ion (HSO₄⁻) then attacks the more stable secondary carbocation, yielding propan‑2‑yl hydrogensulfate as the major product (Markovnikov addition).
Protonation at C‑1 gives a secondary carbocation at C‑2, whereas protonation at C‑2 gives a primary carbocation at C‑1.
The secondary carbocation is more stable than the primary carbocation because the additional alkyl group donates electron density inductively, dispersing the positive charge.
The hydrogensulfate ion (HSO₄⁻) then attacks the more stable secondary carbocation, yielding propan‑2‑yl hydrogensulfate as the major product (Markovnikov addition).
Examiner tips
- Use the exact phrase ‘secondary carbocation’ vs ‘primary carbocation’ to show stability reasoning.
- Mention Markovnikov’s rule to justify the final product.
- Show the step‑by‑step sequence: nucleophilic attack, carbocation formation, stability comparison, nucleophile attack.
- Keep the answer concise – 4 short sentences matching the 4 marks.
Common mistakes
- Confusing the positions of protonation (mixing up C‑1 and C‑2).
- Failing to explain why the secondary carbocation is more stable (missing inductive effect).
- Not mentioning Markovnikov’s rule or the role of HSO₄⁻ as the nucleophile.
Mark scheme (4 marks)
- The π electrons of propene act as a nucleophile and attack the electrophilic hydrogen of H₂SO₄, forming a carbocation intermediate.
- Protonation at C-1 gives a secondary carbocation at C-2, whereas protonation at C-2 gives a primary carbocation at C-1.
- The secondary carbocation is more stable than the primary carbocation due to greater inductive electron-donating effect of the additional alkyl group, which disperses the positive charge.
- The hydrogensulfate ion (HSO₄⁻) then attacks the more stable secondary carbocation, yielding propan-2-yl hydrogensulfate as the major product (Markovnikov's rule / Markovnikov addition).
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →