Propanal and propanone are both carbonyl compounds with the molecular formula C₃H₆O. Explain how the structural difference between propanal and propanone accounts for their different reactions with Fehling's solution, and describe what you would observe in each case.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanal is an aldehyde and propanone is a ketone. Both compounds contain a carbonyl group (C=O).
Model answer (5 marks)
Propanal is an aldehyde because the carbonyl carbon is bonded to a hydrogen atom; the carbonyl group is at the end of the chain. Propanone is a ketone because the carbonyl carbon is bonded to two other carbon atoms; the carbonyl group is in the middle of the chain.
The presence of the C–H bond in the aldehyde allows propanal to be oxidised, so it acts as a reducing agent. In Fehling’s solution the Cu²⁺ ions are reduced to Cu₂O, giving a brick‑red/orange‑red precipitate.
In contrast, propanone lacks a C–H bond on the carbonyl carbon and cannot be oxidised under these conditions. Therefore it does not reduce Cu²⁺; the blue colour of the solution remains unchanged.
The presence of the C–H bond in the aldehyde allows propanal to be oxidised, so it acts as a reducing agent. In Fehling’s solution the Cu²⁺ ions are reduced to Cu₂O, giving a brick‑red/orange‑red precipitate.
In contrast, propanone lacks a C–H bond on the carbonyl carbon and cannot be oxidised under these conditions. Therefore it does not reduce Cu²⁺; the blue colour of the solution remains unchanged.
Examiner tips
- State the structural difference first (C–H vs C–C bonds).
- Explain the redox ability of aldehydes vs ketones. Include the colour change or lack thereof. Use the exact terms ‘brick‑red precipitate’ and ‘blue solution’.
- Keep the answer to 5 points – one for each scheme point.
Mark scheme (5 marks)
- Propanal is an aldehyde because its carbonyl group is at the end of the carbon chain (bonded to at least one hydrogen)
- Propanone is a ketone because its carbonyl group is in the middle of the carbon chain (bonded to two carbon atoms on either side)
- Propanal (aldehyde) is oxidised / acts as a reducing agent and reduces the Cu²⁺ ions in Fehling's solution
- Propanal gives a brick-red/orange-red precipitate with Fehling's solution (copper(I) oxide formed)
- Propanone (ketone) cannot be oxidised and therefore does not react with Fehling's solution, so the solution remains blue
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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