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 solution → green solution). A ketone is not oxidised this way — the basis of the tests that tell them apart.
Reduction. Both are reduced by NaBH4 (sodium tetrahydridoborate) in aqueous solution 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 questions
This question is about the structural isomers shown.
Identify the isomer(s) that would react when warmed with acidified potassium dichromate(VI). State the expected observation when acidified potassium dichromate(VI) reacts.
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M1 — Q, R, S, T. 1 mark
M2 — (orange solution) turns green. 1 mark
M2 is independent of M1. Q is a primary alcohol, R and S are secondary alcohols and T is an aldehyde — all are oxidised. P (a tertiary alcohol) and U (a ketone) are not.
Identify the isomer(s) that would react with Tollens’ reagent. State the expected observation when Tollens’ reagent reacts.
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M1 — T. 1 mark
M2 — silver mirror. 1 mark
A grey/black precipitate is also allowed. Only the aldehyde reacts — Tollens’ does not oxidise the alcohols or the ketone.
Parts (c)–(f) of this question move on to esters and spectroscopy — that chemistry lives in organic analysis and carboxylic acids (3.3.9).
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. The overall equation is written with HCN:
CH3CHO + HCN → CH3CH(OH)CN
Aldehydes and unsymmetrical ketones form a mixture of enantiomers this way — the new carbon carries four different groups, and the planar C=O is attacked from both sides equally. A symmetrical ketone such as propanone gives an achiral product (two identical CH3 groups), so there are no enantiomers to mix.
- 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.
Name the product of 3-methylbutan-2-one, (CH3)2CHCOCH3, with KCN followed by dilute acid.
Step 1 — add CN and OH to the carbonyl carbon:
(CH3)2CHCOCH3 → (CH3)2CHC(OH)(CN)CH3
Step 2 — number from the nitrile carbon. The CN carbon is C1 (it counts as part of the chain — the reaction has extended the chain by one carbon). The longest chain through it is four carbons: butanenitrile.
Step 3 — place the substituents. The old carbonyl carbon is C2, so it carries the OH (2-hydroxy) and one methyl; the CH of the old (CH3)2CH group is C3 with the other methyl.
2-hydroxy-2,3-dimethylbutanenitrile
The OH always lands on C2 — nucleophilic addition puts it on the old carbonyl carbon, which sits next to the nitrile carbon.
Interactive — step through both mechanisms
Exam questions
Aqueous NaBH4 reduces aldehydes but does not reduce alkenes. Show the first step of the mechanism of the reaction between NaBH4 and 2-methylbutanal. You should include two curly arrows. Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene.
Show answer
M1 — correct structure of 2-methylbutanal. 1 mark
M2 — two curly arrows and the lone pair on the hydride ion: 1 mark
C2H5 is allowed for CH3CH2. M2 is penalised if wrong partial charges are put on the C=O; the product is ignored.
M3 — the H− ion / nucleophile is attracted to the δ+ carbon. 1 mark
M4 — the C=C is electron-rich. 1 mark
M5 — the H− ion / nucleophile is repelled by the C=C, OR a C=C is only attacked by electrophiles. 1 mark
A student attempted to reduce a sample of 2-methylbutanal but added insufficient NaBH4. The student confirmed that the reduction was incomplete by using a chemical test. Give the reagent and observation for the chemical test.
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Reagent: Tollens’ (reagent) OR ammoniacal silver nitrate OR a description of making Tollens’. 1 mark
Observation: silver mirror / silver precipitate OR a black solid / precipitate / deposit. 1 mark
Fehling’s / Benedict’s solution with a red precipitate (allow orange or brown) also scores. NOT acidified dichromate — the unreacted aldehyde and the alcohol product cannot be told apart with it, because both are oxidised.
Figure 3 shows the reactant species involved in the first step of a mechanism.
Complete Figure 3 to show the structure of the intermediate formed with curly arrows involved in its formation. Give the name of the reaction mechanism.
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M1 — arrow from the lone pair on the hydride to the C. 1 mark
M2 — arrow from the C=O to the O. 1 mark
M3 — the intermediate structure (displayed or abbreviated structures allowed). 1 mark
M4 — nucleophilic addition. 1 mark
Any attempt to show further correct steps is ignored; further incorrect steps are penalised. The list principle applies to M4. Parts (a) and (b) of this question ask the same for an addition–elimination intermediate (carboxylic acid derivatives, 3.3.9) and a nitration intermediate (aromatic chemistry, 3.3.10).
Source: AQA A-Level Chemistry past papers.
Tests for carbonyl compounds
AQA names two tests for telling an aldehyde from a ketone — Tollens’ reagent and Fehling’s solution — and warming with acidified potassium dichromate(VI) does the same job through oxidation. In all three, only the easily-oxidised aldehyde reacts.
| Reagent | Aldehyde | Ketone |
|---|---|---|
| Tollens’ reagent | silver mirror | no change |
| Fehling’s solution | brick-red ppt | no change |
| Acidified K2Cr2O7 | orange soln → green soln | no change |
Exam questions
Propanone (CH3COCH3) reacts with the weak acid HCN to form a hydroxynitrile. This hydroxynitrile is usually made by reaction of propanone with KCN followed by dilute acid, instead of with HCN. State the hazard associated with the use of KCN. Suggest a reason, other than safety, why KCN is used instead of HCN.
Show answer
M1 — toxic / poisonous. 1 mark
M2 — HCN is weak / [CN−] is too low, or the reverse argument: KCN dissociates (better than HCN) to provide the CN− nucleophile. 1 mark
For M1, “can produce toxic fumes/gas” or “corrosive” is allowed.
Outline the mechanism for the reaction of propanone with KCN followed by dilute acid.
Show answer
M1 — cyanide ion with the lone pair on its C and the negative charge, and a curly arrow from the lone pair to the C of the C=O. 1 mark
M2 — curly arrow from the double bond to the O. 1 mark
M3 — the intermediate anion — the new bond must be to the C of the CN. 1 mark
M4 — curly arrow from a lone pair on the O− to H+. 1 mark
M1 is not given if a K–CN bond is shown breaking; M2 is not given if the dipole is drawn incorrectly. The arrow to H+ may instead go to the H of HCN.
Source: AQA A-Level Chemistry past papers.
Identify & react
Bring it together. First find out what you have, then use its reactions.
- Aldehyde or ketone? Tollens’ (silver mirror) or Fehling’s (brick-red precipitate) — 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 quiz — four past-paper questions
Four real AQA multiple-choice questions on this topic, in the style that opens Paper 3. Pick one answer each — the reasoning appears once you commit.
In which conversion does a nucleophile attack the organic reactant?
What is the product when 3-methylbutan-2-one reacts with acidified KCN?
The skeletal formulas of two compounds are shown.
Which method would distinguish between samples of these compounds?
Which reaction results in an overall change in shape around a carbon atom?
Source: AQA A-Level Chemistry past papers.
Oxidising an aldehyde gives the subject of the next topic: 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 solution → green solution); 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: Tollens’ (silver mirror) and Fehling’s (brick-red precipitate) are positive for aldehydes only.