Carbonyl Compounds: Aldehydes and Ketones

This guide covers subtopic 17.1, Aldehydes and ketones, from Topic 17 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Before studying this

This resource assumes Alcohols: Reactions and Oxidation — the primary-alcohol-to-aldehyde and secondary-alcohol-to-ketone oxidation reactions are the main production route here, and this page picks up immediately from where that one leaves off — and the nucleophilic addition and curly-arrow ideas from Organic Mechanisms: An Introduction.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 17

17.1 Aldehydes and ketones — recalling the reactions by which aldehydes and ketones are produced (oxidation of primary alcohols to aldehydes, and secondary alcohols to ketones, using acidified K₂Cr₂O₇ or KMnO₄ and distillation); describing the reduction of aldehydes and ketones by NaBH₄ or LiAlH₄ to alcohols, and their reaction with HCN (KCN catalyst, heat) to produce hydroxynitriles, exemplified by ethanal and propanone; describing the mechanism of this nucleophilic addition; describing the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect carbonyl compounds; deducing whether an unknown carbonyl compound is an aldehyde or a ketone from Fehling’s and Tollens’ reagents and ease of oxidation; deducing the presence of a CH₃CO– group from the iodoform (tri-iodomethane) reaction.

Production and reduction

Production reuses the oxidation reactions from Alcohols: Reactions and Oxidation directly: oxidising a primary alcohol with acidified K₂Cr₂O₇ or KMnO₄, then distilling the mixture, gives an aldehyde (distillation removes it before it can be oxidised further to a carboxylic acid); the same oxidation applied to a secondary alcohol gives a ketone, which resists further oxidation regardless of the conditions.

Reduction runs this in reverse: NaBH₄ or LiAlH₄ reduces an aldehyde back to a primary alcohol, or a ketone back to a secondary alcohol.

Nucleophilic addition with hydrogen cyanide

Aldehydes and ketones react with HCN (using KCN as a source of CN⁻, with heat) to form a hydroxynitrile:

CH₃CHO + HCN → CH₃CH(OH)CN (from ethanal)

CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN (from propanone)

The carbonyl carbon is electrophilic: oxygen’s greater electronegativity polarises the C=O bond, leaving the carbon δ+ and exposed to nucleophilic attack — the opposite polarity to the C=C bond in an alkene, which is why this reaction is nucleophilic addition rather than electrophilic addition.

Mechanism:

Step 1 — the cyanide ion, CN⁻ (a nucleophile), attacks the δ+ carbonyl carbon. A curly arrow starts at a lone pair on the carbon of CN⁻ and points to the carbonyl carbon. At the same time, a second curly arrow starts at the C=O π bond and points to the oxygen atom, forming a negatively charged alkoxide intermediate with the CN group now bonded to what was the carbonyl carbon.

Step 2 — the alkoxide oxygen, now negatively charged, picks up a proton (from HCN or from the surrounding solution) to give the neutral –OH group of the hydroxynitrile product, regenerating CN⁻ to continue the reaction.

Detecting and distinguishing carbonyl compounds

2,4-DNPH reacts with any carbonyl compound (aldehyde or ketone) to give an orange precipitate — a positive result confirms a C=O group is present, but does not distinguish which type.

To tell an aldehyde from a ketone, use their difference in ease of oxidation — an aldehyde has a hydrogen on the carbonyl carbon and is easily oxidised further to a carboxylic acid; a ketone has no such hydrogen and resists oxidation under the same mild conditions:

Test	With an aldehyde	With a ketone
Fehling’s solution (blue Cu²⁺ complex), warmed	brick-red precipitate of Cu₂O forms (aldehyde is oxidised, Cu²⁺ is reduced)	no colour change
Tollens’ reagent (ammoniacal AgNO₃), warmed	a silver mirror forms on the tube (aldehyde is oxidised, Ag⁺ is reduced to Ag)	no reaction

The iodoform (tri-iodomethane) test identifies a CH₃CO– group specifically, in either an aldehyde or a ketone: warming with alkaline I₂(aq) gives a yellow precipitate of CHI₃ and an ion, RCO₂⁻. Both ethanal (CH₃CHO) and propanone (CH₃COCH₃) give a positive result, since both contain a CH₃CO– group directly.

Worked example. Two unlabelled bottles contain propanal, CH₃CH₂CHO, and propanone, CH₃COCH₃. Describe a chemical test, with observations, that distinguishes them, and explain why the iodoform test would not help here.

Warm each with Tollens’ reagent: propanal (an aldehyde) forms a silver mirror; propanone (a ketone) shows no reaction. The iodoform test would not help, because neither compound contains a CH₃CO– group — propanal’s carbonyl carbon is attached to a CH₂ group, not a CH₃ group, and propanone’s carbonyl carbon is attached to two CH₃ groups but is itself the carbonyl carbon, not adjacent to it — so both would need to be checked against the actual structural requirement (a methyl group directly bonded to the carbonyl carbon) rather than assumed from the general “ketone” label. Propanone in fact does satisfy this (CH₃–CO–CH₃) and gives a positive iodoform result — the point of the example is to check the structure carefully rather than assume based on functional group alone.

Common mistakes

• Using 2,4-DNPH to distinguish an aldehyde from a ketone. It only confirms a carbonyl group is present at all — Fehling’s, Tollens’ or ease of oxidation is needed to tell which type.
• Drawing the nucleophilic addition mechanism with the curly arrow starting from the carbon of CN⁻ pointing the wrong way, or starting the C=O curly arrow from the carbon instead of the π bond. Both arrows must start at electron pairs (a lone pair, or the π bond) and point to where those electrons end up.
• Assuming every ketone gives a negative iodoform test. Whether the test is positive depends on the specific structure — whether a CH₃CO– group is actually present — not on “aldehyde vs ketone” as a category.
• Forgetting that reduction needs the alcohol type to match. An aldehyde reduces to a primary alcohol; a ketone reduces to a secondary alcohol — never the other way round.

Quick revision checklist

• Production: primary alcohol (distil) → aldehyde; secondary alcohol (distil) → ketone
• Reduction with NaBH₄/LiAlH₄ back to the corresponding alcohol
• Nucleophilic addition of HCN: full two-step mechanism, with correct curly arrows
• 2,4-DNPH: confirms a carbonyl group, doesn’t distinguish aldehyde/ketone
• Fehling’s and Tollens’: positive (brick-red / silver mirror) for aldehydes only
• Iodoform test: positive for a CH₃CO– group specifically, in either functional group

Related resources

• Alcohols: Reactions and Oxidation — the oxidation reactions that produce aldehydes and ketones
• Carboxylic Acids and Esters — where further oxidation of an aldehyde leads
• Nitrogen Compounds: Amines and Nitriles — hydroxynitriles’ onward hydrolysis to hydroxy-acids
• Organic Mechanisms: An Introduction — nucleophile, electrophile and curly-arrow notation
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.