Butanone is a carbonyl compound used as a solvent in industry. A student adds a small piece of sodium carbonate to a sample of butanoic acid and to a separate sample of butanone. Explain why butanoic acid reacts with sodium carbonate but butanone does not.

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

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

Butanone is a carbonyl compound belonging to the ketone family. Butanoic acid is a carboxylic acid with four carbon atoms. Both compounds contain a carbonyl group (C=O), but only one belongs to the carboxylic acid family.

Model answer (5 marks)

Butanoic acid is a carboxylic acid and contains a –COOH group. The –COOH group is acidic and can donate a proton to give H⁺ ions in solution. When sodium carbonate (Na₂CO₃) is added, the carbonate ion (CO₃²⁻) accepts a proton to form bicarbonate (HCO₃⁻) and then CO₂ gas:

COOH + CO₃²⁻ → CO₂↑ + HCO₃⁻ + H⁺

The release of CO₂ produces effervescence.

Butanone is a ketone and contains only a C=O group. Ketones are not acidic and do not donate H⁺ ions. Therefore the carbonate ion cannot react, so no CO₂ is evolved and the mixture remains unchanged.

Examiner tips

  • Mention the –COOH group and its acidity first; link to proton donation. Explain the carbonate reaction step‑by‑step, including CO₂ evolution. State that ketones lack the –COOH group and are not acidic, so no reaction. Use the exact terminology: carboxylic acid, carbonate, bicarbonate, effervescence.

Common mistakes

  • Saying that butanone is acidic or that it reacts with carbonate. Forgetting to mention CO₂ evolution or effervescence. Using vague terms like ‘reacts’ without specifying the acid–base mechanism.

Mark scheme (5 marks)

  1. Butanoic acid is a carboxylic acid / contains a –COOH group
  2. Carboxylic acids are acidic / produce H⁺ ions in solution / react with carbonates
  3. Carbon dioxide / CO₂ is produced (bubbles/effervescence observed) when butanoic acid reacts with sodium carbonate
  4. Butanone is a ketone / does not contain a –COOH group / only contains C=O
  5. Butanone is not acidic / does not react with sodium carbonate because ketones do not produce H⁺ ions

Key terms in this question

carbonyl compound · sodium carbonate

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