Aldehydes and ketones
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
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
Describe a chemical test to distinguish propanal from propanone. Give the reagent and the observation with each.
State the reagent used to reduce propanone, name the type of alcohol formed and give the product.
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?
KCN followed by dilute acid reacts with ethanal. Name the mechanism, give the product and explain why the product is a racemic mixture.
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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Stuck on aldehydes and ketones?
The planar-carbonyl racemate answer is where level-3 marks are won or lost — I drill exactly this 'why' answer in lessons, and the first one is free.