AQA A-Level Chemistry Paper 3, June 2025: Question 13

✅ Correct Answer

D: Propene

Summary of the pathway:

CH₃CH=CH₂ (X) + H₂SO₄ → CH₃CH(OSO₂OH)CH₃ (Y)

CH₃CH(OSO₂OH)CH₃ + H₂O → CH₃CH(OH)CH₃ + H₂SO₄

Compound Y is isopropyl hydrogen sulfate (or propyl hydrogen sulfate), which hydrolyses to produce propan-2-ol (an alcohol).

💡 Key Knowledge

• Step 1 (Electrophilic Addition): Alkenes react with cold concentrated H₂SO₄. The electrophile is the partially positive hydrogen atom in H-OSO₃H . Addition follows Markovnikov's rule, producing a secondary alkyl hydrogen sulfate as the major product.
• Step 2 (Hydrolysis): When water is added and the mixture is warmed, the -OSO₂OH (hydrogen sulfate) group is substituted by an -OH group.
• Catalytic Role: Because H₂SO₄ is regenerated at the end, the overall process is the acid-catalysed hydration of an alkene:
  Alkene + H₂O → Alcohol

Concentrated sulfuric acid acts as an electrophile:

CH₃-CH=CH₂ + H₂SO₄ → CH₃-CH(OSO₂OH)-CH₃

Mechanism note: The C=C double bond attacks the electrophilic H atom of H-OSO₃H. A secondary carbocation intermediate ( CH₃-CH⁺-CH₃ ) forms preferentially over the primary carbocation due to inductive stabilisation by two alkyl groups. The ⁻OSO₃H ion then attacks the carbocation.

Hydrolysis with excess warm water:

CH₃-CH(OSO₂OH)-CH₃ + H₂O → CH₃-CH(OH)-CH₃ + H₂SO₄

The sulfate group leaves, yielding the secondary alcohol propan-2-ol and regenerating sulfuric acid.

❌ Analysis of Incorrect Distractors

• A: Ethanoyl chloride — Reacts vigorously directly with water to produce ethanoic acid ( CH₃COOH ) and HCl, not an alcohol. Concentrated H₂SO₄ does not yield an alcohol intermediate.
• B: Methylbenzene — Undergoes electrophilic substitution (sulfonation) with concentrated/fuming H₂SO₄ to form methylbenzenesulfonic acid. Hydrolysis with water does not yield an alcohol.
• C: Propanone — A ketone does not react with concentrated H₂SO₄ followed by water to yield an alcohol. Concentrated acid can act as a catalyst for aldol condensation, not alcohol production.

🧠 Exam Technique & Examiner Insights

• Look for the "Two-Step Hydration": In Year 12 alkene chemistry, industrial hydration uses steam and an acid catalyst ( H₃PO₄ ), but the laboratory method uses concentrated H₂SO₄ followed by excess water. Recognising this laboratory sequence immediately points to an alkene.
• Identify Functional Groups Fast: When an exam question specifies a sequence like X + conc. H₂SO₄ → Y and Y + H₂O → alcohol , mentally replace X with every homologous series. Only alkenes form alkyl hydrogen sulfates that hydrolyse to alcohols.