AQA A-Level Chemistry Paper 3, 2019: Question 21
1 mark · Medium difficulty · Multiple Choice
Identify which reaction mechanism is not used in the multi-step synthesis of benzyl ethanoate from methylbenzene.
Practise this questionQuestion
Question text
21 A possible synthesis of a compound found in jasmine flower oil is shown.
Which mechanism is not used in this synthesis?
[1 mark]
A Electrophilic substitution
B Nucleophilic substitution
C Free-radical substitution
D Nucleophilic addition-elimination
Mark scheme
Show the mark scheme
21 A 1
How to answer it
Organic Synthesis: Identifying Reaction Mechanisms
This question assesses your ability to deduce organic reaction mechanisms across a multi-step synthetic pathway. Specifically, it tests recognition of:
- Side-chain halogenation of alkylarenes via free-radical substitution vs. ring substitution.
- Conversion of halogenoalkanes to alcohols via nucleophilic substitution.
- Acylation of alcohols with acyl chlorides/anhydrides via nucleophilic addition-elimination.
- Careful observation that the aromatic ring (benzene ring) itself remains completely intact throughout the pathway.
Question 21 Breakdown
A multi-step synthesis of benzyl ethanoate from methylbenzene
✅ Correct Answer
A: Electrophilic substitution
📐 Step-by-Step Reaction Pathway Analysis
C₆H₅CH₃ + Cl₂ → C₆H₅CH₂Cl + HCl (reagents: Cl₂, UV light / heat)
Substitution occurs exclusively on the aliphatic -CH₃ side chain, not on the aromatic ring. This follows the free-radical substitution mechanism (option C is used).
C₆H₅CH₂Cl + OH⁻ → C₆H₅CH₂OH + Cl⁻ (reagents: warm aqueous NaOH)
The hydroxide ion acts as a nucleophile attacking the electron-deficient C attached to chlorine, displacing the chloride ion. This is nucleophilic substitution (option B is used).
C₆H₅CH₂OH + CH₃COCl → CH₃COOCH₂C₆H₅ + HCl (reagents: ethanoyl chloride)
The alcohol acts as a nucleophile attacking an acyl chloride (or ethanoic anhydride) to form an ester. The standard A-Level mechanism for this acylation is nucleophilic addition-elimination (option D is used).
💡 Key Knowledge
- Ring vs. Side-chain: Reaction of methylbenzene with chlorine in the presence of an AlCl₃ catalyst gives chloromethylbenzene isomers via electrophilic substitution. However, with UV light and no catalyst, chlorine substitutes into the side-chain via free radicals.
- Intact Benzene Rings: At every step, the 6-carbon aromatic ring retains all 5 aromatic C-H bonds ( C₆H₅- ). No electrophile ever attacks the delocalised pi-system.
🧠 Exam Technique
- Look for the "NOT": The question asks which mechanism is not used. Don't rush and choose the first mechanism you recognise.
- Write mechanisms next to each arrow: In the exam, annotate each arrow with the mechanism name:
• Arrow 1 = FRS
• Arrow 2 = NS
• Arrow 3 = NAE - Elimination method leaves A as the only unused mechanism.
❌ Common Errors
- Assuming benzene means electrophilic substitution: Students see a benzene ring and instinctively pick or rule out electrophilic substitution without verifying if the reaction is on the ring or side-chain.
- Confusing Step 1 conditions: Mistaking side-chain free-radical chlorination for electrophilic ring chlorination.
- Forgetting acylation mechanism name: Students often label esterification simply as "condensation" and forget that acylation via acyl chlorides is explicitly named nucleophilic addition-elimination in the AQA specification.
Topics
Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.2 Alkanes · 3.3.3 Halogenoalkanes · 3.3.9 Carboxylic Acids and Derivatives · 3.3.14 Organic Synthesis
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.