Describe a chemical test to distinguish propanal from propanone. Give the reagent and the observation with each.
[3 marks]
Total for this question: 3
Chemistry
Aldehydes and ketones
AQA 3.3.8
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
The topic on one screen
- Both contain the carbonyl group C=O; aldehydes have it at the end of the chain (-CHO), ketones in the middle.
- Nucleophilic addition: the carbonyl carbon is delta-positive and planar, so nucleophiles attack it.
- Reduction with NaBH4: aldehyde → primary alcohol, ketone → secondary alcohol.
- KCN then dilute acid adds HCN across C=O to give a hydroxynitrile — a way to extend the carbon chain by one.
- Adding HCN to an unsymmetrical carbonyl makes a chiral centre; attack from either face of the planar carbon gives a racemic mixture.
- Tests: 2,4-DNPH (orange precipitate) confirms a carbonyl; Tollens' (silver mirror) or Fehling's (brick-red precipitate) then distinguishes the aldehyde.
Where students actually lose marks
Students stated the C=O bond is planar without including the other atoms attached to the carbon. To score, describe the trigonal-planar arrangement of the three groups around the carbonyl carbon, not just 'the bond is planar'.
June 2024 Paper 2 examiner report (Q08.4)
In carbonyl tests the observations were very well known, but which substance actually reacts in each test was less well understood. Learn the pairing: reagent → observation → which compound gives it.
June 2023 Paper 2 examiner report (Q06.3)
For the racemate explanation you must reach the idea that CN- attacks the planar carbon from above or below the plane with equal probability — that is the level-3 discriminator, not just 'a chiral centre forms'.
June 2024 Paper 2 examiner report (Q08.4)
Try it — exam-style
Medium
ORIGINAL · after June 2023 Paper 2 Q06.3
Easy
ORIGINAL
State the reagent used to reduce propanone, name the type of alcohol formed and give the product.
[2 marks]
Total for this question: 2
Medium
ORIGINAL
An unknown organic liquid gives an orange precipitate with 2,4-dinitrophenylhydrazine but no reaction with Tollens' reagent. What can you conclude about its functional group?
[2 marks]
Total for this question: 2
Hard
ORIGINAL · after June 2024 Paper 2 Q08.4
KCN followed by dilute acid reacts with ethanal. Name the mechanism, give the product and explain why the product is a racemic mixture.
[4 marks]
Total for this question: 4
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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