AQA A-Level Chemistry Paper 3, June 2025: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify which reaction involving potassium hydroxide (KOH) does not involve it acting as a base.

Practise this question

Question

Multiple-choice question asking: 'Each of the conversions shown uses KOH as a reactant. In which conversion does KOH not act as a base?' Option A: (CH3)2NH2+Br- to (CH3)2NH. Option B: CH3COOH to CH3COO-K+. Option C: CH3CH2Br to H2C=CH2. Option D: CH3Br to CH3OH.
Question text

08 Each of the conversions shown uses KOH as a reactant.

In which conversion does KOH not act as a base?

[1 mark]

A (CH ) NH +Br – to (CH ) NH

32 2 3 2

B CH COOH to CH COO–K+

C CH3CH2Br to H2C=CH2

D CH3Br to CH3OH

Mark scheme

Show the mark scheme Mark scheme entry showing for question 08: the correct answer is D, with 1 mark (AO3), noting 'CH3Br to CH3OH'.

08 D 1 (AO3) CH3Br to CH3OH

How to answer it

Role of Potassium Hydroxide: Base vs Nucleophile

📌 What this question tests

This question assesses your understanding of chemical roles across organic and inorganic topics: distinguishing between a Brønsted-Lowry base (a proton/H⁺ acceptor) and a nucleophile (an electron pair donor targeting an electron-deficient carbon). You must evaluate multiple reaction mechanisms involving potassium hydroxide (KOH / OH⁻).

Question 08 Analysis

Identifying where KOH does NOT act as a base

✅ Correct Answer: D

D: CH₃Br to CH₃OH

In this conversion, the hydroxide ion (OH⁻) uses a lone pair of electrons to attack the slightly positive carbon atom (δ+ C) and displaces the bromide ion (Br⁻). Because OH⁻ donates an electron pair to a carbon atom, it is acting as a nucleophile, not a base.

Award 1 mark (AO3) for selecting option D.

💡 Key Knowledge: Base vs Nucleophile

  • Brønsted-Lowry Base: A species that accepts a proton (H⁺).
  • Nucleophile: An electron-pair donor that attacks an electron-deficient centre (typically a δ+ carbon atom).
  • Dual Role of OH⁻: Hydroxide ions can act either as a base or a nucleophile depending on conditions and substrate structure.

📐 Option-by-Option Breakdown

Option Reaction Role of OH⁻ Explanation
A (CH₃)₂NH₂⁺Br⁻ → (CH₃)₂NH Base Deprotonation of dialkylammonium ion: OH⁻ accepts H⁺ to regenerate the secondary amine and form H₂O.
B CH₃COOH → CH₃COO⁻K⁺ Base Neutralisation of ethanoic acid: OH⁻ removes H⁺ from the carboxylic acid group (-COOH) to form water and potassium ethanoate.
C CH₃CH₂Br → H₂C=CH₂ Base Base-catalysed elimination: In hot ethanolic conditions, OH⁻ attacks an adjacent hydrogen atom (H⁺), triggering elimination of HBr to yield ethene.
D CH₃Br → CH₃OH Nucleophile Nucleophilic substitution: OH⁻ attacks the δ+ carbon of bromomethane, displacing Br⁻ to produce methanol. It donates an electron pair to C, NOT H⁺.

❌ Common Student Errors

  • Confusing elimination (C) with substitution: Many students forget that in elimination reactions of haloalkanes to alkenes, OH⁻ acts purely as a base by removing a proton from an adjacent carbon.
  • Overlooking the definition of a base: Students often mistakenly think any reaction with an organic halogen makes OH⁻ a nucleophile, ignoring the elimination mechanism.
  • Misreading the question: Missing the bold word "not", causing students to pick the first obvious acid-base reaction (like B).

🧠 Exam Technique: "Target Atom" Rule

When asked whether OH⁻ acts as a base or a nucleophile, identify the exact atom being attacked by the hydroxide lone pair:

  • Attacking a Hydrogen atom (H⁺) = acting as a BASE (Options A, B, C).
  • Attacking a Carbon atom (δ+ C) = acting as a NUCLEOPHILE (Option D).

Topics

Organic Chemistry · 3.3.3 Halogenoalkanes · 3.3.9 Carboxylic Acids and Derivatives · 3.3.11 Amines

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.