Pentan-2-one is a carbonyl compound used as a flavouring agent in the food industry. A student wants to distinguish between pentan-2-one and pentanoic acid using simple chemical tests. Describe the chemical tests the student could carry out, the results expected for each compound, and explain why the two compounds give different results.

Eduqas A-Level Chemistry — 3.4 Carbonyls and acids · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Pentan-2-one belongs to the ketone family of carbonyl compounds. Pentanoic acid is a carboxylic acid with five carbon atoms.

Model answer (5 marks)

Add a few drops of sodium carbonate (or sodium bicarbonate) solution to a small sample of each compound.

Pentanoic acid:
- Effervescence is observed.
- Bubbles of CO₂ are released.

Pentan‑2‑one:
- No visible reaction; no bubbles.

Explanation:
- Pentanoic acid is a carboxylic acid; it donates H⁺ to carbonate, forming a salt, water and CO₂.
- Pentan‑2‑one is a neutral ketone; it lacks the acidic –COOH group and therefore does not react with carbonate.

Examiner tips

  • Show the two tests clearly; mention the effervescence for the acid and the lack of reaction for the ketone.
  • Explain the acid–base reaction for the acid and the lack of acidity for the ketone.
  • Use the exact terms – ‘carbon dioxide’, ‘carbonate’, ‘carboxylic acid’ and ‘ketone’.
  • Include the reason why the ketone is neutral and does not react.

Common mistakes

  • Confusing the test with a reaction with sodium hydroxide, which would give a different result.
  • Forgetting to mention the CO₂ gas or the lack of bubbles.
  • Using vague terms like ‘acidic’ instead of specifying the –COOH group and the carbonate reaction.

Mark scheme (5 marks)

  1. Add sodium carbonate (or sodium hydrogencarbonate) solution to each compound
  2. Pentanoic acid produces bubbles / fizzing / effervescence with sodium carbonate; carbon dioxide gas is given off
  3. Pentan-2-one shows no reaction / no bubbles with sodium carbonate
  4. Pentanoic acid reacts because it is acidic / it donates H⁺ ions / it reacts with the carbonate to form a salt, water and carbon dioxide
  5. Pentan-2-one does not react because it is not acidic / it does not contain the carboxyl (–COOH) group / ketones are neutral

Key terms in this question

carbonyl compound

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