17.1 A2 Level

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.

Key definition

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

2025-2027 syllabus, in our words · A2 Level

What the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).

  1. Recall how aldehydes and ketones are made, with reagents and conditions: (a) aldehydes, by oxidising primary alcohols with acidified K₂Cr₂O₇ or acidified KMnO₄ and distilling; (b) ketones, by oxidising secondary alcohols with the same reagents and distilling.
  2. Describe: (a) reduction of aldehydes and ketones to alcohols by NaBH₄ or LiAlH₄; (b) how aldehydes and ketones react with HCN and heat, using KCN as a catalyst, to form hydroxynitriles, using ethanal and propanone as examples.
  3. Describe the nucleophilic addition mechanism for the reaction of HCN with aldehydes and ketones (the hydroxynitrile-forming reaction above).
  4. Describe how the reagent 2,4-DNPH (2,4-dinitrophenylhydrazine) is used to test for carbonyl compounds.
  5. Deduce whether an unknown carbonyl compound is an aldehyde or a ketone from simple test results: Fehling's reagent, Tollens' reagent and how easily it is oxidised.
  6. Use the reaction with alkaline I₂(aq), which gives a yellow precipitate of tri-iodomethane and the ion RCO₂⁻, as evidence that an aldehyde or ketone contains the CH₃CO- group, i.e. that it has the structure CH₃CO-R.

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.

aldehydes ketones reduction hydroxynitriles nucleophilic addition 2,4-dinitrophenylhydrazine Fehling's reagent Tollens' reagent

Tips to avoid common mistakes in Aldehydes and ketones

Definition Flip

What reagents and conditions are used to reduce an aldehyde or ketone to an alcohol?

Answer Flip

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.

Key Concept Flip

Describe the mechanism of the nucleophilic addition reaction of hydrogen cyanide (HCN) with an aldehyde or ketone.

Answer Flip

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.

Definition Flip

What reagent is used to detect the presence of a carbonyl group (C=O)?

Answer Flip

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.

Key Concept Flip

How can you distinguish between an aldehyde and a ketone using simple chemical tests?

Answer Flip

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.

Key Concept Flip

What observation indicates the presence of a CH₃CO– group in an aldehyde or ketone?

Answer Flip

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.

Definition Flip

Give the reaction of ethanal with HCN and KCN (catalyst) with heat. Give name of the organic product.

Answer Flip

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.

Definition Flip

Give the reaction of propanone with HCN and KCN (catalyst) with heat. Give name of the organic product.

Answer Flip

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.

More Chemistry flashcards

Browse every 9701 flashcard topic by syllabus area.

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16.1 Alcohols 18.1 Carboxylic acids

Key terms covered in this Aldehydes and ketones deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

What reagents and conditions are used to reduce an aldehyde or ketone to an alcohol
What reagent is used to detect the presence of a carbonyl group (C=O)
Give the reaction of ethanal with HCN and KCN (catalyst) with heat. Give name of the organic product
Give the reaction of propanone with HCN and KCN (catalyst) with heat. Give name of the organic product

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