AQA A-Level Chemistry: Aldehydes and ketones (7405) – Revision Notes

These notes condense section 3.3.8 Aldehydes and ketones of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams from June 2016 onwards. The whole section is A-level only. Organic chemistry (3.3) is examined on A-level Paper 2, and Paper 3 can test any content. For full explanations and worked examples, read the aldehydes and ketones study guide first.

Test yourself afterwards with the aldehydes and ketones practice questions. The course hub is AQA A-Level Chemistry and the printable checklist lists every specification point. For a quick check across the course, use the free 10-minute diagnostics.

Checklist for 3.3.8 (all A-level only)

• Aldehydes are readily oxidised to carboxylic acids.
• Tollens’ reagent and Fehling’s solution distinguish aldehydes from ketones.
• NaBH₄ in aqueous solution: aldehyde → primary alcohol, ketone → secondary alcohol (nucleophilic addition).
• Overall reduction equations with [H].
• Mechanism for NaBH₄ reduction, with H⁻ as the nucleophile.
• KCN followed by dilute acid gives hydroxynitriles; overall equations with HCN.
• Mechanism for KCN followed by dilute acid.
• Why aldehydes and unsymmetrical ketones give a mixture of enantiomers.
• Hazards of using KCN.

Key definitions

Term	Meaning
Carbonyl group	C=O. Polar, Cδ+=Oδ−, because O is more electronegative
Aldehyde	C=O at the end of a chain, –CHO; name ends in -al
Ketone	C=O bonded to two carbons; name ends in -one
Nucleophile	Electron-pair donor; attacks the δ+ carbon
Nucleophilic addition	A nucleophile adds to the C=O carbon and the C=O becomes C–O, then O is protonated
Hydroxynitrile	OH and C≡N on the same carbon; nitrile carbon is C1 in the name
Racemic mixture (racemate)	Equal amounts of two enantiomers; optically inactive

Reactions summary

Reaction	Reagent and conditions	Aldehyde gives	Ketone gives	Type
Oxidation	Acidified K₂Cr₂O₇, or Tollens’, or Fehling’s (warm)	Carboxylic acid (carboxylate in the alkaline reagents)	No reaction	Oxidation
Reduction	NaBH₄ in aqueous solution	Primary alcohol	Secondary alcohol	Nucleophilic addition
Add cyanide	KCN, then dilute acid	Hydroxynitrile	Hydroxynitrile	Nucleophilic addition

Tests (warm in a water bath)

Reagent	Contains	Aldehyde	Ketone
Tollens’	[Ag(NH₃)₂]⁺	Silver mirror (Ag⁺ reduced to Ag)	No change
Fehling’s	Blue Cu²⁺ complex, alkaline	Red precipitate of Cu₂O	Stays blue

Ratios for calculations: 1 mol aldehyde → 2 mol Ag; 1 mol aldehyde → 1 mol Cu₂O (two Cu²⁺ each gain one electron).

Ketones resist oxidation because the C=O carbon has no H; oxidation would need a C–C bond to break.

Equations to learn the shape of

Oxidation:   RCHO + [O] → RCOOH
Reduction:   RCHO + 2[H] → RCH₂OH              (primary alcohol)
             RCOR' + 2[H] → RCH(OH)R'          (secondary alcohol)
Cyanide:     RCHO + HCN → RCH(OH)CN
             RCOR' + HCN → RC(OH)(CN)R'

Examples not used in the study guide:

CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH                       propanal → propan-1-ol
CH₃CH₂COCH₂CH₃ + 2[H] → CH₃CH₂CH(OH)CH₂CH₃           pentan-3-one → pentan-3-ol
CH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN                     → 2-hydroxybutanenitrile
CH₃COCH₂CH₃ + HCN → CH₃C(OH)(CN)CH₂CH₃               → 2-hydroxy-2-methylbutanenitrile

Method in steps: the two mechanisms

Reduction by NaBH₄ (nucleophile H⁻):

1. Show the C=O with δ+ on C and δ− on O.
2. Arrow from the lone pair on H⁻ to the carbonyl carbon.
3. Arrow from the C=O bond to the oxygen. Intermediate: an alkoxide, C–O⁻.
4. Arrow from a lone pair on O⁻ to H of H₂O (or to H⁺). Product: the alcohol.

KCN followed by dilute acid (nucleophile CN⁻):

1. Show the C=O with δ+ on C and δ− on O.
2. Arrow from the lone pair on the carbon of :CN⁻ to the carbonyl carbon.
3. Arrow from the C=O bond to the oxygen. Intermediate: C(CN)–O⁻.
4. Arrow from a lone pair on O⁻ to H⁺. Product: the hydroxynitrile.

Both mechanisms have the same shape: attack on C, electrons to O, then protonate O.

Racemic mixtures in four lines

1. The C=O group is planar (trigonal planar carbon).
2. CN⁻ attacks from either side of the plane with equal probability.
3. If the product carbon has four different groups, the two attacks give the two enantiomers.
4. They form in equal amounts: a racemic mixture, which is optically inactive because the rotations cancel.

Quick rule: an aldehyde (except methanal) or an unsymmetrical ketone gives a chiral product. Methanal (H, H) and symmetrical ketones such as propanone or pentan-3-one do not.

Small worked reminders

Is the product racemic? 3-methylbutan-2-one is CH₃COCH(CH₃)₂.

• Groups on the C=O carbon: CH₃ and CH(CH₃)₂. They are different.
• After KCN then dilute acid, that carbon carries CH₃, CH(CH₃)₂, OH and CN: four different groups.
• So the product is chiral and forms as a racemic mixture.

Which alcohol? Hexanal is an aldehyde, so NaBH₄ gives the primary alcohol hexan-1-ol: CH₃(CH₂)₄CHO + 2[H] → CH₃(CH₂)₄CH₂OH. Hexan-3-one is a ketone, so it gives the secondary alcohol hexan-3-ol.

Which test result? Pentanal with Fehling’s: blue to red precipitate. Pentan-2-one with Fehling’s: stays blue. Pentanal is oxidised to pentanoic acid (as pentanoate in the alkaline reagent).

KCN hazards

• KCN is highly toxic.
• Acid releases HCN, which is highly toxic and volatile.
• Control: fume cupboard, gloves, eye protection, careful addition of acid.
• KCN rather than HCN: HCN is hard to handle safely, and as a weak acid it gives few CN⁻ ions; KCN gives plenty.

Must-know distinctions

Pair	Difference
Tollens’ vs Fehling’s	Silver mirror vs blue → red precipitate
Aldehyde vs ketone reduction product	Primary alcohol vs secondary alcohol
[O] vs [H] equations	[O] adds O (or removes 2H); 2[H] adds two H across C=O
CN⁻ vs H⁻ as nucleophile	CN⁻ adds a carbon (chain grows); H⁻ does not
Chiral product vs racemic mixture	Each molecule is chiral; the 50 : 50 mixture is optically inactive
Symmetrical vs unsymmetrical ketone	Same vs different groups on the C=O carbon; only unsymmetrical gives enantiomers

Related pages: alcohols (making aldehydes and ketones) and organic analysis (test order and IR).

Quick self-test

1. Name the product when 2-methylpropanal is reduced by NaBH₄.
2. Give the class of alcohol formed when a ketone is reduced by NaBH₄.
3. Write the overall equation, using [H], for the reduction of propanal.
4. What is the nucleophile in the reaction of a ketone with KCN, and which atom donates the lone pair?
5. Name CH₃CH₂CH(OH)CN.
6. Which of these gives a racemic product with KCN then dilute acid: methanal, pentan-2-one, pentan-3-one?
7. Why is the racemic product optically inactive?
8. 0.660 g of ethanal (Mr 44.0) reacts fully with Tollens’ reagent. Calculate the mass of silver formed (Ar Ag = 107.9).
9. 1.80 g of butanal (Mr 72.0) is reduced completely. Calculate the maximum mass of butan-1-ol (Mr 74.0).
10. State the observation when hexan-2-one is warmed with Fehling’s solution.
11. Give one reason KCN is used rather than HCN.

Answers

1. 2-methylpropan-1-ol, (CH₃)₂CHCH₂OH.
2. Secondary alcohol.
3. CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH.
4. The cyanide ion, CN⁻; the lone pair is on the carbon atom.
5. 2-hydroxybutanenitrile.
6. Pentan-2-one only (CH₃ and C₃H₇ on the C=O carbon are different).
7. It contains equal amounts of the two enantiomers, which rotate plane-polarised light equally in opposite directions, so the effects cancel.
8. n(ethanal) = 0.660 ÷ 44.0 = 0.0150 mol; n(Ag) = 0.0300 mol; mass = 0.0300 × 107.9 = 3.24 g.
9. n(butanal) = 1.80 ÷ 72.0 = 0.0250 mol; mass = 0.0250 × 74.0 = 1.85 g.
10. No change: the solution stays blue (it is a ketone).
11. HCN is toxic and volatile, so it is hard to handle; or HCN is a weak acid and supplies few CN⁻ ions.

Where marks are usually lost

• CN⁻ arrow drawn from the N atom instead of the lone pair on C.
• Missing the arrow from the C=O bond to O, leaving a five-bonded carbon.
• Showing BH₄⁻ as the attacking species when H⁻ is asked for.
• One [H] instead of 2[H] in reduction equations.
• Forgetting to protonate the O⁻ intermediate in the last step.
• Naming hydroxynitriles without counting the nitrile carbon.
• Explaining racemic mixtures without “planar” and “attack from either side equally”.
• Claiming propanone or methanal gives enantiomers.
• Using a 1 : 1 ratio for aldehyde to silver instead of 1 : 2.
• “Toxic” for KCN with no mention of HCN release on adding acid.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.8 Aldehydes and ketones (A-level only).