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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cyclohexanone is a cyclic ketone used as an industrial solvent. Its carbonyl group is part of the six-membered carbon ring.
Model answer (5 marks)
Cyclohexanone reacts with 2,4‑DNPH to give an orange/yellow precipitate of the corresponding hydrazone, showing that a carbonyl group is present. The DNPH test does not distinguish between aldehydes and ketones, so it only confirms that the compound is a carbonyl.
When the same sample is treated with Fehling’s solution the solution remains blue; no brick‑red Cu₂O precipitate is formed. Fehling’s reagent oxidises aldehydes to carboxylic acids, giving a red precipitate of Cu₂O, but ketones are not oxidised under these conditions. The absence of a colour change therefore indicates that the compound is a ketone, not an aldehyde.
When the same sample is treated with Fehling’s solution the solution remains blue; no brick‑red Cu₂O precipitate is formed. Fehling’s reagent oxidises aldehydes to carboxylic acids, giving a red precipitate of Cu₂O, but ketones are not oxidised under these conditions. The absence of a colour change therefore indicates that the compound is a ketone, not an aldehyde.
Examiner tips
- Mention the orange/yellow DNPH precipitate first, then state it only confirms C=O. Next describe the blue Fehling’s solution and lack of red precipitate, linking this to the inability of ketones to be oxidised. Conclude that the negative Fehling’s result proves the compound is a ketone.
Common mistakes
- Saying the DNPH test distinguishes aldehydes from ketones. "No change with Fehling’s” is often omitted or incorrectly described as a colour change. Confusing the colour of the DNPH precipitate (orange/yellow) with the Fehling’s red precipitate.
Mark scheme (5 marks)
- Cyclohexanone reacts with 2,4-DNPH to form an orange/yellow precipitate
- The 2,4-DNPH test confirms the presence of a carbonyl group (C=O) but cannot distinguish between an aldehyde and a ketone
- Cyclohexanone produces no colour change / no brick-red precipitate with Fehling's solution (remains blue)
- Ketones cannot be oxidised (by Fehling's solution), so no Cu₂O precipitate is formed
- 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 in this question
carbonyl compound · aldehyde · ketone · 2,4-dinitrophenylhydrazine (2,4-DNPH) · Fehling's solution
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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