Butanone and butanal are both carbonyl compounds with the molecular formula C₄H₈O. A chemist wants to identify which compound is which using Tollens' reagent. Describe what the chemist would observe with each compound and explain why the two compounds give different results.

Eduqas A-Level Chemistry — 4.4 Aldehydes and ketones · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Butanal will give a silver mirror (or silver precipitate) when treated with Tollens' reagent. Butanone will show no change – no silver is deposited.

This occurs because butanal is an aldehyde and can be oxidised; the aldehyde group reduces the Ag⁺ ions in Tollens' reagent to metallic silver, forming the mirror. In contrast, butanone is a ketone and is not oxidised under these conditions, so it cannot reduce the Ag⁺ ions and no silver is formed.

Examiner tips

  • Use the command words – ‘describe’ the visual change, ‘explain’ the redox reason.
  • Mention the aldehyde/ketone distinction and the role of Ag⁺ reduction.
  • Show the silver mirror as the key observation for butanal.

Common mistakes

  • Confusing the reaction of butanal with a precipitate instead of a silver mirror.
  • Saying that both compounds give a silver mirror or that ketones can be oxidised by Tollens' reagent.

Mark scheme (5 marks)

  1. Butanal produces a silver mirror (or silver precipitate) with Tollens' reagent
  2. Butanone produces no change / no reaction with Tollens' reagent
  3. Butanal is an aldehyde and can be oxidised (acts as a reducing agent)
  4. The silver ions in Tollens' reagent are reduced to silver metal by butanal
  5. Butanone is a ketone and cannot be oxidised under these conditions, so it does not reduce Tollens' reagent

Key terms in this question

Tollens' reagent

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