What is Tollens' reagent?

Ammoniacal silver nitrate, containing [Ag(NH₃)₂]⁺.

What is observed when an aldehyde is warmed with Tollens' reagent?

A silver mirror forms as Ag⁺ is reduced to silver.

What is observed when a ketone is warmed with Tollens' reagent?

No change; there is no silver mirror.

What is observed when an aldehyde is warmed with Fehling's solution?

The blue solution gives a red precipitate of copper(I) oxide.

Write the overall equation for reducing ethanal using [H].

CH₃CHO + 2[H] → CH₃CH₂OH

Why is the carbon of C=O attacked by nucleophiles?

Oxygen is more electronegative than carbon, so the C=O bond is polar and the carbon is δ+.

What forms after H⁻ attacks the carbonyl carbon?

An alkoxide intermediate with a negative charge on oxygen, which is then protonated by water.

Reactions of aldehydes and ketonesAQA A-Level Chemistry: Flashcards

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Which carbonyl compounds are readily oxidised?

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Which carbonyl compounds are readily oxidised?: Aldehydes, to carboxylic acids. Ketones are not easily oxidised.
What is Tollens' reagent?: Ammoniacal silver nitrate, containing [Ag(NH₃)₂]⁺.
What is observed when an aldehyde is warmed with Tollens' reagent?: A silver mirror forms as Ag⁺ is reduced to silver.
What is observed when a ketone is warmed with Tollens' reagent?: No change; there is no silver mirror.
What is observed when an aldehyde is warmed with Fehling's solution?: The blue solution gives a red precipitate of copper(I) oxide.
What is observed when a ketone is warmed with Fehling's solution?: The solution stays blue.
What does NaBH₄ reduce an aldehyde to?: A primary alcohol.
What does NaBH₄ reduce a ketone to?: A secondary alcohol.
What species is the nucleophile in reduction with NaBH₄?: The hydride ion, H⁻.
Write the overall equation for reducing ethanal using [H].: CH₃CHO + 2[H] → CH₃CH₂OH
Why is the carbon of C=O attacked by nucleophiles?: Oxygen is more electronegative than carbon, so the C=O bond is polar and the carbon is δ+.
What forms after H⁻ attacks the carbonyl carbon?: An alkoxide intermediate with a negative charge on oxygen, which is then protonated by water.

Exam questions on Reactions of aldehydes and ketones

A technician has two unlabelled colourless liquids, one of which is propanal and the other propanone. She warms a sample of each separately with Tollens' reagent and with Fehling's solution in a hot water bath.
State what is observed when propanone is tested with each reagent, and explain why propanal reacts differently.2 marks
A chemist reduces a sample of butanal and a sample of butanone separately, using NaBH₄ in aqueous solution at room temperature.
Write an overall equation for the reduction of butanal, using [H] to represent the reductant, and name the type of mechanism.2 marks
Two isomeric carbonyl compounds, pentanal and pentan-2-one, are each treated with NaBH₄ in aqueous solution. The organic products are then separated and identified.
Identify the organic product formed from each carbonyl compound, and state the class of alcohol in each case.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).