# A student reacts cyclohexanone with a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH). The student then attempts to use Fehling's solution to determine whether cyclohexanone is an aldehyde or a ketone. Explain what the student would observe in each reaction, and explain why these observations allow or do not allow a conclusion about the type of carbonyl compound present.

> Edexcel A-Level Chemistry (9CH0) — 17.2 Aldehydes and ketones · Explain · 5 marks

> Cyclohexanone is a cyclic ketone used as an industrial solvent. Its carbonyl group is part of the six-membered carbon ring.

## Mark scheme (5 marks)

1. Cyclohexanone reacts with 2,4-DNPH to form an orange/yellow precipitate
2. The 2,4-DNPH test confirms the presence of a carbonyl group (C=O) but cannot distinguish between an aldehyde and a ketone
3. Cyclohexanone produces no colour change / no brick-red precipitate with Fehling's solution (remains blue)
4. Ketones cannot be oxidised (by Fehling's solution), so no Cu₂O precipitate is formed
5. The negative Fehling's result confirms cyclohexanone is a ketone, not an aldehyde, because an aldehyde would have produced a brick-red precipitate

## Key terms

- [carbonyl compound](https://www.gradenine.co.uk/glossary/carbonyl-compound)
- [aldehyde](https://www.gradenine.co.uk/glossary/aldehyde)
- [ketone](https://www.gradenine.co.uk/glossary/ketone)
- [2,4-dinitrophenylhydrazine (2,4-DNPH)](https://www.gradenine.co.uk/glossary/2-4-dinitrophenylhydrazine-2-4-dnph)
- [Fehling's solution](https://www.gradenine.co.uk/glossary/fehling-s-solution)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-reacts-cyclohexanone-with-a-ee48b8f0) · Published by Druglandscape Ltd.