AQA A-Level Chemistry Paper 3, June 2025: Question 36

AQA A-Level Chemistry • Organic Mechanisms

Hydration of 1-Methylcyclopent-1-ene & Carbocation Stability

What this question tests

This question tests your ability to apply the mechanism of electrophilic addition (acid-catalysed hydration of an unsymmetrical cyclic alkene). Specifically, it assesses understanding of carbocation stability (Markovnikov's rule), the identity of intermediate species, and distinguishing the pathway leading to the major product versus the minor product.

Question 36 (Multiple Choice)

Identifying the Intermediate Ion Leading to the Major Product

✅ Correct Answer

A (The oxonium ion / protonated alcohol intermediate at C1)

Mark Scheme: A (1 mark, AO2)

💡 Key Knowledge

Alkene structure: 1-methylcyclopent-1-ene has a double bond between C1 (attached to a methyl group) and C2 (a -CH- group).
Electrophilic attack: H⁺ adds to C2, producing a tertiary carbocation at C1. Tertiary carbocations are more stable than secondary carbocations due to the positive inductive effect (+I) of three alkyl substituents.
Major vs. Minor: The major product forms via the more stable tertiary carbocation at C1.
Oxonium intermediate: Water acts as a nucleophile, using a lone pair on oxygen to attack the C1 carbocation, forming a protonated alcohol intermediate ( -⁺OH₂ ).

📐 Step-by-Step Mechanism Breakdown

Step 1: Protonation across the C=C bond
An electrophilic H⁺ from H₂SO₄ attacks the alkene.
- Pathway 1 (leads to major product): H⁺ adds to C2. The positive charge resides on C1, generating a tertiary carbocation.
- Pathway 2 (leads to minor product): H⁺ adds to C1. The positive charge resides on C2, generating a secondary carbocation (shown in option B).
Step 2: Nucleophilic attack by water
In Pathway 1, a lone pair from H₂O attacks the positive C1 centre. This directly forms the protonated alcohol intermediate (Structure A), where oxygen bears a single positive formal charge ( -C(CH₃)-⁺OH₂ ).
Step 3: Deprotonation
Loss of H⁺ yields the neutral major product: 1-methylcyclopentan-1-ol, regenerating the acid catalyst.

❌ Why Other Options Are Incorrect

B is the minor pathway intermediate: Structure B shows a secondary carbocation at C2 with the methyl group at C1. This forms via the less stable pathway and yields the minor product (2-methylcyclopentan-1-ol), not the major product.
C is an incorrect formula: The conjugate base of H₂SO₄ is the hydrogen sulfate ion, HSO₄⁻ (or ⁻OSO₂OH). ⁻SO₂OH is chemically incorrect (it represents a sulfinate-type species).
D cannot exist: ⁻OH (hydroxide ion) cannot form or exist in an acidic reaction mixture containing concentrated sulfuric acid.

🧠 Exam Technique & Examiner Tips

Read the word "major" carefully: Many students hastily choose B simply because they recognise a carbocation. Always check whether the carbocation shown is the more stable or less stable intermediate!
Classify the carbons: C1 is bonded to three carbons (tertiary), while C2 is bonded to two carbons (secondary). Tertiary is always favoured.
Remember that intermediates include oxonium ions: In hydration, the carbocation is only the first intermediate; the protonated alcohol ( -⁺OH₂ ) is also an ionic intermediate formed during the mechanism.