Tollens' reagent and Fehling's solution are both used to distinguish between aldehydes and ketones. Describe what is observed when each type of compound is tested with Tollens' reagent, and explain why ketones do not react with these oxidising agents.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Aldehydes give a bright silver mirror (or silver precipitate) when treated with Tollens' reagent.
Ketones show no visible change – no silver mirror or precipitate.
The reason is that Tollens' reagent is an oxidising agent. It oxidises aldehydes to the corresponding carboxylic acids, reducing the silver ion (Ag⁺) to metallic silver.
Ketones cannot be oxidised under these conditions because the carbonyl carbon has no hydrogen atom attached; it is bonded to two carbon groups, so the oxidation step that removes a hydrogen cannot occur.
Ketones show no visible change – no silver mirror or precipitate.
The reason is that Tollens' reagent is an oxidising agent. It oxidises aldehydes to the corresponding carboxylic acids, reducing the silver ion (Ag⁺) to metallic silver.
Ketones cannot be oxidised under these conditions because the carbonyl carbon has no hydrogen atom attached; it is bonded to two carbon groups, so the oxidation step that removes a hydrogen cannot occur.
Examiner tips
- Mention the silver mirror for aldehydes and the lack of reaction for ketones. Explain oxidation to carboxylic acid and the role of Ag⁺ reduction. Highlight the absence of a hydrogen on the carbonyl carbon in ketones.
Common mistakes
- Saying that ketones give a silver mirror. Forgetting to state that aldehydes are oxidised to carboxylic acids. Confusing the reason for ketone inertness with steric hindrance rather than lack of hydrogen.
Mark scheme (5 marks)
- Aldehydes produce a silver mirror (or silver precipitate) with Tollens' reagent
- Ketones show no change / no reaction with Tollens' reagent
- Aldehydes are oxidised (by the Tollens' reagent / oxidising agent)
- Aldehydes are oxidised to carboxylic acids
- Ketones cannot be oxidised under these conditions because the carbonyl carbon has no hydrogen atom directly attached to it (i.e. it is bonded to two carbon groups)
Key terms in this question
aldehyde · ketone · Tollens' reagent
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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