Describe how aldehydes and ketones can be distinguished using acidified potassium dichromate(VI).

Aldehydes can be oxidised to carboxylic acids when refluxed with acidified potassium dichromate. During this process, the solution turns from orange to green.

Ketones, however, do not undergo oxidation with acidified potassium dichromate so the solution remains orange.

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What is the general equation for the reduction of a ketone, RCOR', with a reducing agent, [H]?

RCOR' + 2[H] ➔ RCH(OH)R'

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Name 2 reducing agents used in the reduction of aldehydes and ketones to alcohols.

  1. Sodium borohydride (NaBH4)
  2. Lithium aluminium hydride (LiAlH4)

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Outline the mechanism for the reaction above.

Ketones also react with CN- via the same mechanism.

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Name the mechanism in the reaction between carbonyl compounds and hydrogen cyanide.

Nucleophilic addition

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Why is the reaction between carbonyl compounds and hydrogen cyanide useful?

This reaction is useful as it provides a means of increasing the length of the carbon chain by 1 carbon atom.

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What type of product is formed in the reaction of an aldehyde or ketone with hydrogen cyanide?

A hydroxynitrile is formed.


The two functional groups of a hydroxynitrile are a hydroxyl group (-OH) and a nitrile group (C≡N).

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How and why is hydrogen cyanide generated in situ for the reaction with aldehydes and ketones?


Hydrogen cyanide is generated in situ for the reaction with aldehydes and ketones by mixing potassium cyanide or sodium cyanide with a dilute acid. This is done because HCN is a toxic gas, and generating it in situ allows for safer handling and use in the reaction.

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What is the general equation for the reaction of an aldehyde (RCHO) with hydrogen cyanide (HCN)?

RCHO + HCN ➔ RCH(OH)CN

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Why do carbonyl compounds react with nucleophiles?

The C=O bond is polarised, with the carbon being partially positive (and the more electronegative oxygen being partially negative). The partially positive carbon is suspectible to attack by nucleophiles.

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Why do carbonyl compounds undergo addition reactions?

Carbonyl compounds are unsaturated because they contain a C=O double bond - this means they can undergo addition reactions.

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