Aldehydes and ketones
Cambridge A-Level Chemistry (9701) · Unit 17: Carbonyl compounds · 7 flashcards
Aldehydes and ketones is topic 17.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 17 — Carbonyl compounds . In one line: Aldehydes and ketones are reduced to alcohols using either NaBH₄ (sodium borohydride) in aqueous ethanol or LiAlH₄ (lithium aluminum hydride) in dry ether, followed by dilute acid. NaBH₄ is generally used for aldehydes and ketones; LiAlH₄ is a stronger reducing agent and more suitable for reducing carboxylic acids and other functionalities as well.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 4 definitions and 3 key concepts — covering the precise wording mark schemes reward. Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What reagents and conditions are used to reduce an aldehyde or ketone to an alcohol
Aldehydes and ketones are reduced to alcohols using either NaBH₄ (sodium borohydride) in aqueous ethanol or LiAlH₄ (lithium aluminum hydride) in dry ether, followed by dilute acid. NaBH₄ is generally used for aldehydes and ketones; LiAlH₄ is a stronger reducing agent and more suitable for reducing carboxylic acids and other functionalities as well.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall the reactions (reagents and conditions) by which aldehydes and ketones can be produced:
- describe: (a) the reduction of aldehydes and ketones using NaBH4 or LiAl H4 to produce alcohols (b) the reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat to produce hydroxynitriles as exemplified by ethanal and propanone
- describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones in 17.1.2(b)
- describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH reagent) to detect the presence of carbonyl compounds
- deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (Fehling’s and Tollens’ reagents; ease of oxidation)
- deduce the presence of a CH3CO – group in an aldehyde or ketone, CH3CO–R, from its reaction with alkaline I2(aq) to form a yellow precipitate of tri-iodomethane and an ion, RCO2 –
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Aldehydes and ketones
- › Construct balanced half-equations to determine electron transfer; remember one mole of dichromate(VI) ions gains six moles of electrons.
- › Always check for chiral centers in isomerism questions; for instance, 2-methylbutanal has two optical isomers.
- › Ensure the molecule contains the -CH(OH)CH3 or -COCH3 group at both ends to produce a dicarboxylic acid via the iodoform reaction.
What reagents and conditions are used to reduce an aldehyde or ketone to an alcohol?
Aldehydes and ketones are reduced to alcohols using either NaBH₄ (sodium borohydride) in aqueous ethanol or LiAlH₄ (lithium aluminum hydride) in dry ether, followed by dilute acid. NaBH₄ is generally used for aldehydes and ketones; LiAlH₄ is a stronger reducing agent and more suitable for reducing carboxylic acids and other functionalities as well.
Describe the mechanism of the nucleophilic addition reaction of hydrogen cyanide (HCN) with an aldehyde or ketone.
The nucleophilic cyanide ion (CN⁻) attacks the partially positive carbon atom of the carbonyl group (C=O). The pi bond breaks, and electrons move to the oxygen, forming an alkoxide ion. This ion is then protonated by H⁺ from HCN to form a hydroxynitrile.
What reagent is used to detect the presence of a carbonyl group (C=O)?
2,4-Dinitrophenylhydrazine (2,4-DNPH) reagent is used to detect the presence of a carbonyl group. A positive result is indicated by the formation of a yellow, orange, or red precipitate.
How can you distinguish between an aldehyde and a ketone using simple chemical tests?
Aldehydes can be oxidized easily, whereas ketones require stronger conditions. Use Tollen's reagent (silver mirror test) and Fehling's solution (brick red precipitate) to distinguish. Aldehydes will react positively with both, while ketones will not.
What observation indicates the presence of a CH₃CO– group in an aldehyde or ketone?
A yellow precipitate of tri-iodomethane (CHI₃) forms when the aldehyde or ketone is reacted with alkaline iodine (I₂/NaOH). This reaction occurs because the CH₃CO– group is present, producing the characteristic yellow solid.
Give the reaction of ethanal with HCN and KCN (catalyst) with heat. Give name of the organic product.
Ethanal reacts with HCN in the presence of KCN (catalyst) and heat to produce 2-hydroxypropanenitrile (or ethanal cyanohydrin). The cyanide ion attacks the carbonyl carbon.
Give the reaction of propanone with HCN and KCN (catalyst) with heat. Give name of the organic product.
Propanone reacts with HCN in the presence of KCN (catalyst) and heat to produce 2-hydroxy-2-methylpropanenitrile (or propanone cyanohydrin). The cyanide ion attacks the carbonyl carbon.
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