Aldehydes and ketones are both carbonyl compounds. Explain how you could use Tollens' reagent to distinguish between an aldehyde and a ketone, and explain why the test gives different results for each compound class.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Tollens' reagent contains silver ions in aqueous ammonia solution. It is used to distinguish between aldehydes and ketones.
Model answer (5 marks)
Aldehydes give a positive result with Tollens' reagent – a silver mirror forms on the inside of the test tube.
Ketones give a negative result – no silver mirror or colour change is observed.
The test works because an aldehyde is a reducing agent. In the presence of the alkaline silver ion complex, the aldehyde is oxidised to a carboxylate ion while the silver ion is reduced to metallic silver:
RCHO + 2Ag⁺ + 2OH⁻ → RCOO⁻ + 2Ag(s) + H₂O
A ketone lacks a hydrogen on the carbonyl carbon and therefore cannot be oxidised under these mild conditions; consequently it does not reduce Ag⁺ and no silver is deposited.
Thus the differing ability of aldehydes and ketones to act as reducing agents gives the distinct results.
Ketones give a negative result – no silver mirror or colour change is observed.
The test works because an aldehyde is a reducing agent. In the presence of the alkaline silver ion complex, the aldehyde is oxidised to a carboxylate ion while the silver ion is reduced to metallic silver:
RCHO + 2Ag⁺ + 2OH⁻ → RCOO⁻ + 2Ag(s) + H₂O
A ketone lacks a hydrogen on the carbonyl carbon and therefore cannot be oxidised under these mild conditions; consequently it does not reduce Ag⁺ and no silver is deposited.
Thus the differing ability of aldehydes and ketones to act as reducing agents gives the distinct results.
Examiner tips
- Use the command word ‘explain’ – describe the reaction and the reason for the different outcomes. Include the key points: silver mirror for aldehydes, none for ketones, reduction of Ag⁺, oxidation of aldehyde, lack of C–H bond in ketones. Mention the reaction equation to show the redox process.
- Show the mechanism briefly: aldehyde → carboxylate + Ag⁰, ketone → no change.
- Use UK spelling (e.g. ‘mirrors’ if plural).
Common mistakes
- Writing that both aldehydes and ketones give a silver mirror. Forgetting to mention that the aldehyde is oxidised to a carboxylate. Using the wrong reaction equation (e.g. reducing to alcohol instead of silver).
Mark scheme (5 marks)
- Aldehydes give a positive result with Tollens' reagent / silver mirror forms on the inside of the tube with an aldehyde
- Ketones give a negative result / no silver mirror / no change observed with Tollens' reagent
- Aldehydes are oxidised (by the Tollens' reagent) / aldehyde acts as a reducing agent
- Silver ions (Ag⁺) are reduced to silver metal (Ag) by the aldehyde
- Ketones cannot be oxidised under these mild conditions / ketones do not have the C–H bond on the carbonyl carbon and so cannot act as reducing agents
Key terms in this question
aldehyde · ketone · Tollens' reagent · carbonyl
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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