Butanal and butanone are both carbonyl compounds. A chemist wants to identify which of two unlabelled bottles contains butanal and which contains butanone. Describe how the chemist could use Fehling's solution to distinguish between the two compounds, and explain the results that would be observed.

WJEC A-Level Chemistry (Wales) — 4.4 Aldehydes and ketones · Describe · 5 marks · View as Markdown

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

Butanal has the molecular formula C₄H₈O and belongs to the aldehyde family. Butanone has the molecular formula C₄H₈O and belongs to the ketone family.

Model answer (5 marks)

Add Fehling’s solution to a small sample of each compound and heat the mixtures.

For the aldehyde (butanal) the Fehling’s solution is reduced; the aldehyde is oxidised to a carboxylic acid and the copper(II) ions are reduced to Cu₂O, giving a brick‑red/orange‑red precipitate.

For the ketone (butanone) no reaction occurs; the ketone cannot be oxidised under these conditions, so the blue Cu²⁺ solution remains unchanged.

Thus the bottle that gives a red precipitate contains butanal, while the bottle that remains blue contains butanone.

Examiner tips

  • Show the step of adding Fehling’s solution and heating; mention the red precipitate for aldehyde and no change for ketone; explain oxidation of aldehyde and lack of oxidation of ketone.
  • Use the exact terms ‘brick‑red precipitate’, ‘Cu₂O’, ‘Fehling’s solution remains blue’.
  • Keep the answer concise and to the point – 5 marks are awarded for each point listed in the scheme.

Common mistakes

  • Confusing the colour of the precipitate (e.g. writing ‘yellow’ instead of ‘brick‑red’).
  • Failing to mention that the blue solution remains unchanged for the ketone.
  • Not stating that the aldehyde is oxidised and the ketone is not oxidised under these conditions.

Mark scheme (5 marks)

  1. Add Fehling's solution to a sample of each compound and heat / warm the mixture
  2. Butanal (the aldehyde) produces a brick-red / orange-red precipitate
  3. Butanone (the ketone) produces no change / Fehling's solution remains blue
  4. Aldehydes are oxidised / butanal is oxidised (by Fehling's solution)
  5. Ketones cannot be oxidised (under these conditions) / ketones do not react with Fehling's solution

Key terms in this question

Fehling's solution

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