OxfordAQA A-Level Chemistry: Aldehydes and ketones (9620) – Revision Notes

These notes condense section 3.3.8 (subsection 3.3.8.1), Aldehydes and ketones, of the OxfordAQA International AS and A-level Chemistry (9620) specification (Version 5.3; International AS and A-level exams May/June 2020 onwards). Its content is International A-level only, and the unit that examines it is Unit 4 (Organic 2 and Physical 2); Unit 5, the practical and synoptic unit, can also use it. The aldehydes and ketones study guide explains each point in full with worked examples; begin there if anything below is unclear.

When you are ready, try the practice questions. The course hub and the printable checklist span the whole course; the free 10-minute diagnostics help you pick what to revise next.

3.3.8.1 at a glance

Learning outcome	One-line summary
Oxidation	Aldehydes are readily oxidised to carboxylic acids; ketones are not
Tests	Fehling’s: blue → red precipitate with an aldehyde. Tollens’: silver mirror with an aldehyde
NaBH₄ reduction	Aqueous NaBH₄: aldehyde → primary alcohol, ketone → secondary alcohol
Equations	Use 2[H] for reduction and HCN for hydroxynitrile formation
Mechanisms	Nucleophilic addition; H⁻ for NaBH₄, CN⁻ for KCN then dilute acid
Enantiomers	Aldehydes (not methanal) and unsymmetrical ketones give a racemate with KCN then dilute acid
Hazards	KCN is highly toxic; acid releases HCN gas

No practical activity is listed under 3.3.8.1. The same tests feature in Required practical 5 (section 3.3.6.1); see the organic analysis notes.

Key words

Term	What to write
Carbonyl group	C=O; polar, Cδ+=Oδ−, because oxygen is more electronegative
Aldehyde	C=O carbon bonded to at least one H; –CHO; suffix -al
Ketone	C=O carbon bonded to two carbons; suffix -one
Nucleophile	Lone-pair donor that bonds to an electron-deficient carbon
Nucleophilic addition	Nucleophile bonds to the C=O carbon; the double-bond electrons shift to O; O⁻ gains H⁺
Hydroxynitrile	One carbon carrying both –OH and –C≡N; count the nitrile carbon as C1
Racemate	Equal amounts of two enantiomers; optically inactive

Reactions table

Reagent	Conditions	Aldehyde →	Ketone →	Reaction type
Fehling’s solution	Warm, water bath	Carboxylate; red Cu₂O	No change (blue)	Oxidation of aldehyde
Tollens’ reagent	Warm, water bath	Carboxylate; silver mirror	No change	Oxidation of aldehyde
NaBH₄	Aqueous solution	Primary alcohol	Secondary alcohol	Nucleophilic addition
KCN	Then add dilute acid	Hydroxynitrile	Hydroxynitrile	Nucleophilic addition

Mole ratios for calculations: 1 aldehyde : 2 Ag (Tollens’); 1 aldehyde : 1 Cu₂O (Fehling’s). The silver half-equation is [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃ (section 3.2.5.5 names this reduction as the test).

Equation patterns

RCHO + [O] → RCOOH              oxidation of an aldehyde
RCHO + 2[H] → RCH₂OH            reduction to a primary alcohol
RCOR′ + 2[H] → RCH(OH)R′        reduction to a secondary alcohol
RCHO + HCN → RCH(OH)CN          hydroxynitrile from an aldehyde
RCOR′ + HCN → RC(OH)(CN)R′      hydroxynitrile from a ketone

New examples to practise on:

CH₃CH₂C(CH₃)₂CHO + [O] → CH₃CH₂C(CH₃)₂COOH          2,2-dimethylbutanoic acid
(CH₃)₃CCOCH₂CH₃ + 2[H] → (CH₃)₃CCH(OH)CH₂CH₃         2,2-dimethylpentan-3-ol
(C₂H₅)₂CHCH₂CH₂CHO + 2[H] → (C₂H₅)₂CHCH₂CH₂CH₂OH     4-ethylhexan-1-ol
CH₃CO(CH₂)₇CH₃ + HCN → CH₃C(OH)(CN)(CH₂)₇CH₃         2-hydroxy-2-methyldecanenitrile

Method in steps: the mechanisms

NaBH₄ (nucleophile H⁻)

1. Label Cδ+ and Oδ− on the C=O.
2. Arrow: lone pair of H⁻ → carbonyl carbon.
3. Arrow: C=O bond → oxygen. Intermediate: alkoxide, –C–O⁻.
4. Arrow: O⁻ lone pair → H of water (or H⁺). Product: alcohol.

KCN then dilute acid (nucleophile CN⁻)

1. Label Cδ+ and Oδ− on the C=O.
2. Arrow: lone pair on the C of :CN⁻ → carbonyl carbon.
3. Arrow: C=O bond → oxygen. Intermediate: –C(CN)–O⁻.
4. Arrow: O⁻ lone pair → H⁺. Product: hydroxynitrile.

Same skeleton both times: attack carbon, push electrons to oxygen, protonate oxygen.

Racemate checklist

To explain a mixture of enantiomers, make all four points:

1. The C=O carbon is trigonal planar.
2. CN⁻ can attack either face of the plane.
3. Each face is attacked with equal probability.
4. If the product carbon has four different groups, equal amounts of the two enantiomers form, and their equal and opposite rotations cancel.

Quick screen: list the two groups on the C=O carbon. Different → racemate. Identical (methanal; symmetrical ketones such as undecan-6-one) → no chiral centre, so no enantiomers.

Small worked reminders

Which test result? 2,2-dimethylpentan-3-one with Tollens’ reagent: no silver mirror (ketone). 4-ethylhexanal with Tollens’: silver mirror, giving 4-ethylhexanoate.

Racemate? Decan-2-one carries CH₃ and C₈H₁₇ on its C=O carbon. They differ, so its hydroxynitrile forms as a racemate.

Naming. In CH₃C(OH)(CN)(CH₂)₇CH₃, count from the nitrile carbon: C1 (CN), C2 (C–OH), then eight more carbons along the octyl chain makes ten in all. So the name is 2-hydroxy-2-methyldecanenitrile.

KCN safety in four lines

• KCN: highly toxic solid.
• With acid: forms HCN, a highly toxic, volatile gas.
• Controls: fume cupboard, gloves, eye protection, slow addition of dilute acid.
• KCN rather than HCN: KCN supplies many CN⁻ ions; HCN is a weak acid and a dangerous gas.

Must-know distinctions

Pair	Key difference
Fehling’s vs Tollens’	Blue solution → red precipitate vs silver mirror
Aldehyde vs ketone with NaBH₄	Primary vs secondary alcohol
[H] vs HCN equations	2[H] adds H to C and to O; HCN adds H to O and CN to C
H⁻ vs CN⁻	CN⁻ lengthens the carbon chain by one; H⁻ does not
Chiral molecule vs racemate	Every molecule is chiral, but equal amounts cancel, so the mixture is optically inactive
Symmetrical vs unsymmetrical ketone	Identical vs different groups on the C=O carbon

Related notes: alcohols (making aldehydes and ketones) and optical isomerism (enantiomers and racemates).

Quick self-test

Ar values: Ag 107.9, Cu 63.5, O 16.0, C 12.0, H 1.0.

1. Name the product when 2,2-dimethylbutanal is warmed with acidified potassium dichromate(VI).
2. What is seen when 2,2-dimethylpentan-3-one is warmed with Fehling’s solution?
3. Write the [H] equation for the reduction of 2,2-dimethylpentan-3-one and name the product.
4. In the NaBH₄ mechanism, what species is the nucleophile, and where does the first curly arrow end?
5. Write the overall HCN equation for decan-2-one.
6. Which atom of the cyanide ion donates the lone pair?
7. Does undecan-6-one give a racemate with KCN then dilute acid? Explain briefly.
8. Give two reasons the KCN reaction must be done in a fume cupboard with care over adding the acid.
9. 2.28 g of 2,2-dimethylpentan-3-one (C₇H₁₄O) is fully reduced. Calculate the maximum mass of alcohol (C₇H₁₆O).
10. 1.60 g of 4-ethylhexanal (C₈H₁₆O) reacts with excess Tollens’ reagent. Calculate the mass of silver.
11. 0.0400 mol of an aldehyde reacts fully with Fehling’s solution. Calculate the mass of Cu₂O.

Answers

1. 2,2-dimethylbutanoic acid.
2. No change: the solution stays blue (no red precipitate), because it is a ketone.
3. (CH₃)₃CCOCH₂CH₃ + 2[H] → (CH₃)₃CCH(OH)CH₂CH₃; 2,2-dimethylpentan-3-ol.
4. The hydride ion, H⁻; the arrow ends on the carbonyl carbon.
5. CH₃CO(CH₂)₇CH₃ + HCN → CH₃C(OH)(CN)(CH₂)₇CH₃.
6. The carbon atom.
7. No. Its C=O carbon carries two identical pentyl groups, C₅H₁₁, so the product carbon does not have four different groups.
8. KCN is highly toxic; adding acid releases HCN, which is a highly toxic, volatile gas.
9. Mr: 114.0 and 116.0. n = 2.28 ÷ 114.0 = 0.0200 mol; mass = 0.0200 × 116.0 = 2.32 g.
10. Mr = 128.0; n = 1.60 ÷ 128.0 = 0.0125 mol; n(Ag) = 0.0250 mol; mass = 0.0250 × 107.9 = 2.70 g.
11. Mr(Cu₂O) = 143.0; mass = 0.0400 × 143.0 = 5.72 g.

Where marks are usually lost

• Beginning the cyanide arrow at N; the attacking lone pair belongs to C.
• Drawing BH₄⁻ as the nucleophile instead of H⁻.
• No arrow from the double bond on to oxygen.
• Stopping at the alkoxide, with no protonation step.
• Writing [H] instead of 2[H] in a reduction equation.
• Using 1 : 1 instead of 1 : 2 for aldehyde to silver.
• Dropping the nitrile carbon when naming a hydroxynitrile.
• Explaining a racemate as “the product is chiral” with no mention of a planar group and equal attack from either side.
• Giving “no reaction” instead of an observation for the ketone in a test.
• Listing KCN hazards without HCN formation on adding acid.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.3.8 Aldehydes and ketones (3.3.8.1), International A2 content.