Butanone and butanal are both carbonyl compounds. Explain why butanone cannot be oxidised by acidified potassium dichromate(VI) solution, whereas butanal can, and describe what would be observed in each case when the reagent is added.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Butanone has the molecular formula CH₃COCH₂CH₃. Butanal has the molecular formula CH₃CH₂CH₂CHO.
Model answer (5 marks)
Butanal is an aldehyde; it has a hydrogen atom directly bonded to the carbonyl carbon (–CHO). The acidified potassium dichromate(VI) oxidises this C–H bond, converting the aldehyde to a carboxylic acid and the dichromate ion is reduced from orange to green.
Butanone is a ketone; both sides of the carbonyl carbon are carbon chains, so there is no C–H bond on the carbonyl carbon. Therefore the reagent cannot oxidise the carbonyl group and the orange colour of the dichromate remains unchanged.
Thus, with butanal the solution turns green; with butanone it stays orange.
Butanone is a ketone; both sides of the carbonyl carbon are carbon chains, so there is no C–H bond on the carbonyl carbon. Therefore the reagent cannot oxidise the carbonyl group and the orange colour of the dichromate remains unchanged.
Thus, with butanal the solution turns green; with butanone it stays orange.
Examiner tips
- Identify the functional group (aldehyde vs ketone) and the presence/absence of a C–H bond on the carbonyl carbon; link this to the redox change of dichromate (orange→green).
- State the observed colour change for each compound explicitly.
Mark scheme (5 marks)
- Butanal is an aldehyde, so it has a hydrogen atom directly bonded to the carbonyl carbon (CHO group)
- This hydrogen can be removed, allowing the aldehyde to be oxidised to a carboxylic acid
- Butanone is a ketone, so it has no hydrogen atom on the carbonyl carbon; carbon chains are on both sides of the C=O group
- With butanal: the orange/orange-brown acidified potassium dichromate(VI) solution turns green
- With butanone: the acidified potassium dichromate(VI) solution remains orange — no colour change occurs
Key terms in this question
oxidised · acidified potassium dichromate(VI)
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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