AQA A-Level Chemistry Paper 3, June 2018: Question 10

1 mark · Medium difficulty · Multiple Choice

Identify the correct reaction and mechanism statement for a cycloalkene derivative containing bromo and aldehyde functional groups.

Practise this question

Question

Multiple choice question showing a cyclic organic molecule with a six-membered ring containing two C=C double bonds, a bromine atom attached to an sp3 ring carbon between two methyl-substituted alkene carbons, and an aldehyde group (-CHO) on the opposite side of the ring. Four options describe reactions: A reacting with HBr via electrophilic substitution, B reacting with NaBH4 via nucleophilic addition-elimination, C reacting with ethanolic KOH via elimination, and D reacting with KCN via nucleophilic substitution.
Question text

10 Which statement is correct about the molecule shown?

[1 mark]

A It reacts with HBr in an electrophilic substitution reaction.

B It reacts with NaBH4 in a nucleophilic addition-elimination reaction.

C It reacts with ethanolic KOH in an elimination reaction.

D It reacts with KCN in a nucleophilic substitution reaction.

Mark scheme

Show the mark scheme Mark scheme table indicating for question 10 that the correct answer is option D.

10 D

How to answer it

Organic Reaction Mechanisms & Functional Groups

📌 What this question tests

This question assesses your ability to inspect a polyfunctional organic molecule, identify the reactive centres, and correctly match each functional group with its corresponding reaction type and mechanism:

  • Haloalkane chemistry: Nucleophilic substitution reactions with cyanide ions (:CN⁻) vs elimination reactions with hydroxide (:OH⁻).
  • Carbonyl chemistry: Reduction of aldehydes using aqueous/ethanolic NaBH₄ via nucleophilic addition.
  • Alkene chemistry: Addition reactions across non-aromatic C=C double bonds with hydrogen halides (electrophilic addition).
  • Structural awareness: Recognising degree of substitution and lack of β-hydrogen atoms preventing certain pathways.

Question 10 Breakdown

Identifying the Correct Reaction Statement

✅ Correct Option: D

"It reacts with KCN in a nucleophilic substitution reaction."

The molecule contains a secondary haloalkane carbon ( C–Br ). Cyanide ions ( :CN⁻ ) act as nucleophiles, attacking the partially positive carbon ( Cδ+–Brδ- ) and displacing the bromide leaving group in a nucleophilic substitution reaction to form a nitrile.

💡 Functional Group Mapping

Identify all 3 functional groups present in the molecule:

  • Aldehyde ( –CHO ): Undergoes nucleophilic addition (e.g. reduction by NaBH₄, or addition of HCN/KCN).
  • Alkenes (two unconjugated/isolated C=C bonds): Undergo electrophilic addition with HBr or Br₂. Note: This ring is not benzene (it is a cyclohexa-1,4-diene derivative).
  • Bromoalkane ( –CH(Br)– ): Undergoes nucleophilic substitution with nucleophiles such as :CN⁻, :NH₃, and :OH⁻.

❌ Why Options A, B, and C are Incorrect

  • A is incorrect: The ring has isolated alkene ( C=C ) double bonds, not an aromatic benzene ring. Reaction with HBr proceeds via electrophilic addition, not electrophilic substitution.
  • B is incorrect: NaBH₄ reduces the aldehyde ( –CHO ) to a primary alcohol via nucleophilic addition. Only acyl chlorides and acid anhydrides undergo nucleophilic addition-elimination.
  • C is incorrect: Elimination of HBr requires a hydrogen on an adjacent (β) carbon. The two adjacent ring carbons are each bonded to a methyl group, a double bond, and the C–Br carbon (total = 4 bonds), leaving no β-hydrogens on the ring to be eliminated to form a conjugated diene/allene system.

🧠 Exam Strategy: Mechanism Names

AQA frequently constructs distractors by pairing a plausible-sounding reagent with the wrong mechanism type:

  • Alkene + HBr → always Electrophilic Addition
  • Aldehyde/Ketone + NaBH₄ → always Nucleophilic Addition
  • Acyl chloride + nucleophile → Nucleophilic Addition-Elimination
  • Haloalkane + KCN → Nucleophilic Substitution

Always check the mechanism descriptor word-by-word!

Examiner Insight: Students often rush and see "NaBH₄" and "aldehyde" and immediately assume Option B is correct, failing to spot the word "elimination". Always read the mechanism classification carefully: aldehydes never undergo addition-elimination under A-Level specifications.

Topics

Organic Chemistry · 3.3.1 Introduction to Organic Chemistry · 3.3.3 Halogenoalkanes · 3.3.4 Alkenes · 3.3.8 Aldehydes and Ketones

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.