What suffix names a ketone, and how is its position shown?

-one, with a number for the C=O carbon (e.g. pentan-2-one).

Why can aldehydes and ketones not hydrogen bond with themselves?

They have no hydrogen atom bonded to oxygen.

Why are small aldehydes and ketones soluble in water?

The lone pairs on the carbonyl oxygen accept hydrogen bonds from water molecules.

What do Fehling's solution and an aldehyde give on warming?

The blue solution forms a brick-red precipitate of copper(I) oxide. Ketones give no change.

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

A silver mirror, as Ag⁺ is reduced to Ag. Ketones give no change.

What is the colour change when acidified dichromate(VI) oxidises an aldehyde?

Orange to green (Cr₂O₇²⁻ reduced to Cr³⁺).

What does LiAlH₄ in dry ether do to an aldehyde and to a ketone?

Aldehyde → primary alcohol; ketone → secondary alcohol (RCHO + 2[H] → RCH₂OH).

Why must the ether be dry when using LiAlH₄?

LiAlH₄ reacts violently with water.

Outline the mechanism of HCN addition to a carbonyl compound.

CN⁻ attacks the δ+ carbon using its lone pair; the C=O π bond breaks to give an O⁻ intermediate; O⁻ takes H⁺ from HCN to give the hydroxynitrile.

Why does HCN with ethanal give a product with no optical activity?

The planar C=O is attacked equally from both sides, giving a racemic mixture of two enantiomers.

What does 2,4-DNPH show, and how can it identify a carbonyl compound?

An orange precipitate shows C=O; recrystallise it and compare its melting temperature with data-book values.

What does iodine in alkali test for, and what is seen?

The CH₃CO– group; pale yellow precipitate of triiodomethane, CHI₃.

Aldehydes and ketonesEdexcel A-Level Chemistry: Flashcards

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What is the carbonyl group, and where is it in an aldehyde?

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What is the carbonyl group, and where is it in an aldehyde?: C=O; in an aldehyde it is at the end of the chain, –CHO.
What suffix names a ketone, and how is its position shown?: -one, with a number for the C=O carbon (e.g. pentan-2-one).
Why can aldehydes and ketones not hydrogen bond with themselves?: They have no hydrogen atom bonded to oxygen.
Why are small aldehydes and ketones soluble in water?: The lone pairs on the carbonyl oxygen accept hydrogen bonds from water molecules.
What do Fehling's solution and an aldehyde give on warming?: The blue solution forms a brick-red precipitate of copper(I) oxide. Ketones give no change.
What is seen when Tollens' reagent is warmed with an aldehyde?: A silver mirror, as Ag⁺ is reduced to Ag. Ketones give no change.
What is the colour change when acidified dichromate(VI) oxidises an aldehyde?: Orange to green (Cr₂O₇²⁻ reduced to Cr³⁺).
What does LiAlH₄ in dry ether do to an aldehyde and to a ketone?: Aldehyde → primary alcohol; ketone → secondary alcohol (RCHO + 2[H] → RCH₂OH).
Why must the ether be dry when using LiAlH₄?: LiAlH₄ reacts violently with water.
Outline the mechanism of HCN addition to a carbonyl compound.: CN⁻ attacks the δ+ carbon using its lone pair; the C=O π bond breaks to give an O⁻ intermediate; O⁻ takes H⁺ from HCN to give the hydroxynitrile.
Why does HCN with ethanal give a product with no optical activity?: The planar C=O is attacked equally from both sides, giving a racemic mixture of two enantiomers.
What does 2,4-DNPH show, and how can it identify a carbonyl compound?: An orange precipitate shows C=O; recrystallise it and compare its melting temperature with data-book values.
What does iodine in alkali test for, and what is seen?: The CH₃CO– group; pale yellow precipitate of triiodomethane, CHI₃.

Exam questions on Aldehydes and ketones

A forensic analyst is given two colourless liquids. One is propanal and the other is propanone, both with molecular formula C₃H₆O. She has Tollens' reagent, Fehling's solution, 2,4-dinitrophenylhydrazine solution and acidified potassium dichromate(VI) available.
A positive result with 2,4-dinitrophenylhydrazine shows that both liquids are carbonyl compounds. Describe how the analyst could use this reagent to confirm that one of the liquids is propanone.2 marks
Ethanal reacts with hydrogen cyanide, HCN, in the presence of a small amount of potassium cyanide, KCN, to form a hydroxynitrile. The product contains a carbon atom bonded to four different groups, but the product mixture shows no effect on plane-polarised light.
Explain why the product mixture does not rotate the plane of plane-polarised light.2 marks
A research chemist studies the reduction of propanal and propanone, and also investigates a compound X of molecular formula C₄H₈O. Compound X gives an orange precipitate with 2,4-dinitrophenylhydrazine, gives no change with Tollens' reagent and gives a pale yellow precipitate when warmed with iodine and aqueous sodium hydroxide.
Propanal and propanone are each reduced by lithium tetrahydridoaluminate, LiAlH₄, in dry ether. State the organic product from each and explain why the ether must be dry.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).