You met aldehydes and ketones as the oxidation products of alcohols at AS, and their tests in organic analysis. Here they get their own reactions, all driven by one feature: the polar C=O double bond. Because the carbon is electron-poor (δ+), carbonyl compounds are attacked by nucleophiles — and two nucleophilic-addition mechanisms are the heart of the topic.
The carbonyl group: oxidation & reduction
Aldehydes (R–CHO) and ketones (R–CO–R′) both contain the carbonyl group, C=O. Oxygen is far more electronegative than carbon, so the double bond is polar: the carbon is δ+ and the oxygen δ−. That δ+ carbon is the target for nucleophiles.
Oxidation. An aldehyde is readily oxidised to a carboxylic acid (by warming with acidified potassium dichromate(VI), orange → green). A ketone is not oxidised this way — the basis of the tests that tell them apart.
Reduction. Both are reduced by NaBH4 (sodium tetrahydridoborate) to alcohols: an aldehyde gives a primary alcohol, a ketone a secondary alcohol. Using [H] for the reductant:
CH3CHO + 2[H] → CH3CH2OH
CH3COCH3 + 2[H] → CH3CH(OH)CH3
🧪 Exam-style questions
A student reduces 2-methylbutanal with NaBH4 but adds too little, so the reduction is incomplete. Give a chemical test, with the observation, that confirms an aldehyde is still present.
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Add Tollens’ reagent (or Fehling’s solution) and warm. 1 mark
A silver mirror forms (Tollens’) / a brick-red precipitate forms (Fehling’s) — showing an aldehyde remains. 1 mark
Source: AQA A-Level Chemistry past papers.
Nucleophilic addition
Both key reactions add a nucleophile across the C=O by the same three moves: the nucleophile attacks the δ+ carbon, the C=O double bond breaks onto the oxygen (forming a negative alkoxide), and the oxygen is then protonated. Learn one mechanism and you have both.
Reduction by NaBH4
NaBH4 delivers a hydride ion, H−, which acts as the nucleophile:
- Arrow 1 — from the H− (a B–H bond of BH4−) to the δ+ carbon of C=O.
- Arrow 2 — from the C=O double bond to the oxygen, forming a negatively charged alkoxide intermediate.
- Arrow 3 — a lone pair on the alkoxide O takes an H+ (from water / dilute acid), giving the alcohol.
NaBH4 reduces the C=O but not a C=C double bond: H− is a nucleophile, and only the C=O is polar with a δ+ carbon to attack. A C=C is non-polar and electron-rich, so it does not attract the nucleophile.
Addition of HCN (using KCN)
The cyanide ion, CN−, is the nucleophile. Addition across the C=O gives a hydroxynitrile (a 2-hydroxynitrile), which is one carbon longer than the carbonyl — a useful way to extend a carbon chain.
- Arrow 1 — from a lone pair on the C of CN− to the δ+ carbon of C=O.
- Arrow 2 — from the C=O double bond to the oxygen, forming the alkoxide.
- Arrow 3 — the alkoxide O takes an H+ (from the dilute acid / HCN), giving the hydroxynitrile.
- KCN is very toxic (it releases toxic HCN). It is used instead of HCN because HCN is a volatile, extremely toxic gas and a weak acid, so KCN provides a higher, controlled concentration of the CN− nucleophile.
- Every curly arrow must start from a lone pair or a bond and finish on an atom or bond — the nucleophile’s lone pair to the δ+ carbon, the C=O to the oxygen.
🧪 Exam-style questions
Show the first step of the mechanism of the reaction between NaBH4 and 2-methylbutanal (include two curly arrows), and explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene.
Show answer
Arrow from the H− (hydride) to the δ+ carbon of the C=O. 1 mark
Arrow from the C=O double bond to the oxygen (forming the alkoxide intermediate). 1 mark
The C=O bond is polar, with a δ+ carbon. 1 mark
H− is a nucleophile, so it is attracted to that δ+ carbon. 1 mark
The C=C bond in the alkene is non-polar / electron-rich, so it does not attract the nucleophile — no reaction. 1 mark
Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.
Show answer
Arrow from a lone pair on the C of CN− to the δ+ carbon of the C=O. 1 mark
Arrow from the C=O double bond to the oxygen. 1 mark
Correct alkoxide intermediate with the negative charge on oxygen. 1 mark
Arrow from the O− lone pair to H+ (dilute acid) → 2-hydroxy-2-methylpropanenitrile. 1 mark
Propanone reacts with the weak acid HCN, but the hydroxynitrile is usually made using KCN followed by dilute acid instead. State the hazard of KCN, and suggest a reason (other than safety) why KCN is used instead of HCN.
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Hazard: KCN is (very) toxic / poisonous (it can release toxic HCN). 1 mark
HCN is only a weak acid, so it provides a low concentration of the CN− nucleophile; KCN gives a higher concentration of CN−, so the reaction is faster. 1 mark
Ethanal reacts with KCN then dilute acid to form 2-hydroxypropanenitrile, a mixture of equal amounts of two isomers. Name the mechanism, and name this type of mixture.
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Mechanism: nucleophilic addition. 1 mark
Type of mixture: a racemic mixture (racemate). 1 mark
The planar C=O is attacked from both sides equally, giving the two enantiomers in equal amounts.
Source: AQA A-Level Chemistry past papers.
Tests for carbonyl compounds
Two questions: is it a carbonyl at all, and is it an aldehyde or a ketone? Three reagents answer them.
| Reagent | Aldehyde | Ketone |
|---|---|---|
| 2,4-DNP (Brady’s reagent) | orange ppt | orange ppt |
| Tollens’ reagent | silver mirror | no change |
| Fehling’s solution | brick-red ppt | no change |
| Acidified K2Cr2O7 | orange → green | no change |
2,4-DNP (Brady’s reagent) gives an orange precipitate with any carbonyl — it confirms a C=O. To identify which carbonyl, the precipitate is purified by recrystallisation and its melting point is measured and compared with known values. Tollens’ and Fehling’s then separate aldehydes from ketones: only the easily-oxidised aldehyde gives a positive result.
🧪 Exam-style questions
Identify a reagent that would give a positive result with an aldehyde but not a ketone, and state the observation with the aldehyde.
Show answer
Tollens’ reagent (or Fehling’s solution). 1 mark
Silver mirror forms (Tollens’) / brick-red precipitate forms (Fehling’s). 1 mark
A ketone is reacted with 2,4-DNP to form a crystalline solid, which is then identified by its melting point. Describe how the crystalline solid is separated and purified.
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Separate the solid by filtration (under reduced pressure). 1 mark
Recrystallise: dissolve in the minimum volume of hot solvent, then cool so the pure product crystallises; filter off the crystals. 1 mark
Wash the crystals with a little cold solvent and dry them. 1 mark
Source: AQA A-Level Chemistry past papers.
Identify & react
Bring it together. First find out what you have, then use its reactions.
- Is it a carbonyl? 2,4-DNP gives an orange precipitate for any aldehyde or ketone.
- Aldehyde or ketone? Tollens’ (silver mirror) or Fehling’s (brick-red) — positive for aldehydes only.
- Its reactions: reduce with NaBH4 (→ alcohol), or add HCN/KCN (→ hydroxynitrile, extending the chain).
- Give the reagent and the observation for every test — “Tollens’ → silver mirror”, not just “Tollens’”.
- In the mechanisms, the nucleophile’s arrow starts from its lone pair and the C=O arrow finishes on the oxygen; show the negative alkoxide intermediate.
- A chiral hydroxynitrile forms as a racemate — the planar C=O is attacked from both sides equally.
🧪 Capstone question
Two unlabelled liquids are propanal and propanone. Describe how you would use chemical tests to identify which is which, giving the reagents and observations.
Show answer
Both give an orange precipitate with 2,4-DNP, confirming both are carbonyls. 1 mark
Add Tollens’ reagent (or Fehling’s) and warm. 1 mark
Propanal (the aldehyde) gives a silver mirror (or brick-red precipitate); propanone (the ketone) gives no change. 1 mark
Source: AQA A-Level Chemistry past papers.
Oxidising an aldehyde gives the subject of the next page: carboxylic acids and their derivatives (3.3.9).
- Carbonyl C=O is polar (Cδ+=Oδ−), so it undergoes nucleophilic addition.
- Oxidation: aldehyde → carboxylic acid (acidified K2Cr2O7, orange→green); ketone not oxidised.
- Reduction (NaBH4): nucleophilic addition of hydride (H−) — aldehyde → 1° alcohol; ketone → 2° alcohol.
- HCN / KCN: nucleophilic addition of CN− gives a hydroxynitrile (extends the chain by one carbon); a chiral product forms as a racemate (planar C=O attacked from both sides).
- Tests: 2,4-DNP gives an orange precipitate with any carbonyl (mp identifies which); Tollens’ (silver mirror) and Fehling’s (brick-red) are positive for aldehydes only.