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Topic 3.3.8: Aldehydes and Ketones - Exam Practice

AQA A-Level Chemistry • Organic Chemistry • Exam-Style Questions

AQA A-LEVEL CHEMISTRY

Topic 3.3.8: Aldehydes and Ketones Exam Practice

Test your understanding of C=O properties, reduction mechanisms, Tollens/Fehling distinction, and nucleophilic addition with exam-style questions.

Aldehydes & Ketones › Exam Practice

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Structured Questions

Complete each question on paper, then check your answers against the mark scheme.

Question 1: Nucleophilic Addition Mechanism

5 marks

Write the mechanism for the reaction of ethanal with potassium cyanide, KCN, followed by dilute acid, to form 2-hydroxypropanenitrile. Include all necessary curly arrows, lone pairs, and formal charges. [5]

Show Mark Scheme
  • Show dipole on C=O bond of ethanal (C delta+, O delta-). [1]
  • Curly arrow from the lone pair on the carbon atom of the cyanide ion (:CN-) to the carbonyl carbon atom. [1]
  • Curly arrow from the C=O double bond to the oxygen atom. [1]
  • Correct structure of the intermediate: CH3-C(CN)(O-)-H (tetrahedral carbon, negative charge on oxygen). [1]
  • Curly arrow from the lone pair on the negative oxygen atom to a proton (H+ from the acid) to form the hydroxyl group. [1]
Examiner tip: Ensure the charge and lone pair are on the carbon atom of the :CN- ion, not the nitrogen. The first arrow must start from this lone pair. The second arrow must clearly point to the oxygen, not the C-O region.

Question 2: Reduction of Carbonyl Compounds

4 marks

A chemist reduces butanone using sodium tetrahydridoborate(III), NaBH₄.

(a) Write a balanced chemical equation for this reduction. Use [H] to represent the reducing agent. [1]

(b) Identify the nucleophile in this reaction and draw the first step of the mechanism, showing how it attacks the butanone molecule. [3]

Show Mark Scheme

(a)

  • CH₃COCH₂CH₃ + 2[H] → CH₃CH(OH)CH₂CH₃ [1]

(b)

  • Nucleophile: Hydride ion / H⁻ (with lone pair: :H⁻) [1]
  • Mechanism first step:
    • Show dipole on carbonyl group (C delta+, O delta-). [1]
    • Curly arrow starting from the lone pair on the H⁻ ion to the carbonyl carbon, and a second curly arrow from the C=O double bond to the oxygen atom. [1]
Examiner tip: A common mistake is writing H⁺ as the nucleophile. Remember, a nucleophile is an electron-pair donor, so it must be H⁻ (hydride) with a lone pair.

Question 3: Distinction of Carbonyls

4 marks

Describe how the student could use Fehling's solution to distinguish between samples of propanal and propanone. Write a half-equation showing the reduction of copper(II) ions in the reagent. [4]

Show Mark Scheme
  • Add Fehling's solution to both samples and heat/warm in a water bath. [1]
  • Observation with Propanal (aldehyde): Blue solution forms a red / brick-red precipitate of copper(I) oxide (Cu₂O). [1]
  • Observation with Propanone (ketone): Solution remains blue / no reaction. [1]
  • Half-equation: 2Cu²⁺ + 2OH⁻ + 2e⁻ → Cu₂O + H₂O (or simply: 2Cu²⁺ + H₂O + 2e⁻ → Cu₂O + 2H⁺) [1]
Examiner tip: Ensure you state that heat is required. Reactions of Fehling's and Tollens' are very slow or do not occur at room temperature. A hot water bath is the safest way to heat flammable organic substances.

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