Aldehydes and ketones are two families of organic compounds that contain the carbonyl group. Explain the difference between an aldehyde and a ketone in terms of the position of the carbonyl group, and describe how Tollens' reagent can be used to distinguish between them, including the observations you would expect for each compound type.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
In an aldehyde the carbonyl carbon is at the end of the carbon chain (terminal) and is bonded to at least one hydrogen atom. In a ketone the carbonyl carbon is situated in the middle of the chain and is bonded to two carbon atoms.
Tollens' reagent is an aqueous solution of silver ions (Ag⁺) in ammonia (ammoniacal silver nitrate). When an aldehyde is added it is oxidised and the silver ions are reduced to metallic silver, forming a silver mirror on the inner surface of the test tube – a positive result.
A ketone does not react with Tollens' reagent; the silver ions remain unchanged and no silver mirror is produced – a negative result, the solution stays colourless.
Tollens' reagent is an aqueous solution of silver ions (Ag⁺) in ammonia (ammoniacal silver nitrate). When an aldehyde is added it is oxidised and the silver ions are reduced to metallic silver, forming a silver mirror on the inner surface of the test tube – a positive result.
A ketone does not react with Tollens' reagent; the silver ions remain unchanged and no silver mirror is produced – a negative result, the solution stays colourless.
Examiner tips
- Mention the terminal vs. internal position of the C=O and the H vs. C bonds. State that Tollens' reagent is ammoniacal AgNO₃. Describe the silver mirror as the key observation for aldehydes. Note that ketones give no mirror and the solution stays colourless.
Common mistakes
- Confusing the position of the carbonyl group (e.g., saying both are terminal). Forgetting to specify that the aldehyde must have a hydrogen attached. Misidentifying Tollens' reagent as just silver nitrate, not ammoniacal.
Mark scheme (5 marks)
- In an aldehyde, the carbonyl group (C=O) is at the end of the carbon chain (terminal position) / bonded to at least one hydrogen atom
- In a ketone, the carbonyl group (C=O) is in the middle of the carbon chain / bonded to two carbon atoms on either side
- Tollens' reagent is an aqueous solution of silver ions / silver nitrate in ammonia solution (ammoniacal silver nitrate)
- An aldehyde reduces the silver ions and produces a silver mirror on the inside of the test tube (positive result)
- A ketone does not react with Tollens' reagent and no silver mirror is formed (negative result) / the solution remains colourless
Key terms in this question
carbonyl group · aldehyde · ketone · Tollens' reagent
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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