AQA Chem 4: Carbonyl Compounds, Aldehydes and Ketones

Revision notes on aldehydes and ketones from AQA Unit 4

  • Created by: anna
  • Created on: 04-06-13 11:51



A carbonyl is an organic compound with the C=O functional group.

In aldehydes the carbonyl group is at the end of the chain, their name ending is -al.

In ketones the carbonyl group is in the middle of the chain, their name ending is -one.  

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Fehling's Test:

  • Blue solution containing copper (II) ions changes to a red precipitate in the presence of an aldehyde.
  • Aldehyde is oxidised to carboxylic acid and the copper (II) ions are reduced to copper (I) oxide

Tollen's Reagent:

  • Colourless solution of silver nitrate forms a silver mirror in the presence of an aldehyde
  • Aldehyde oxidised to carboxylic acid, silver ions reduced to silver metal

Acidified Potassium Dichromate:

  • Orange solution of Cr2O72- changes to green solution of Cr3+
  • Aldehyde oxidised to carboxylic acid, dichromate reduced to Cr3+
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Reducing aldehydes and ketones

Using NaBH4:

  • It can be used in aqueous ethanol to reduce aldehydes and ketones
  • It acts as a source of H- ions which attack the C+ on the carbonyl
  • RCHO + 2[H] --> RCH2OH  (aldehyde to primary alcohol)
  • RCOR + 2[H] --> RCH(OH)R  (ketone to secondary alcohol)
  • The mechanism is nucleophilic addition

Using H2 with a nickel catalyst:

  • Using H2 (g) and Ni reduces aldehydes and ketones to the corresponding alcohol
  • It also reduces all other double bonds in the molecule
  • RCHO + H2 --> RCH2OH
  • RCOR + H2 --> RCH(OH)R
  • CH2=CHCHO + 2H2 --> CH3CH2CH2OH
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Reactions of Carbonyls with HCN


  • Mechanism: nucleophilic addition
  • CN nucleophile attacks the C+ on the carbonyl
  • Generally forms a hydroxynitrile
  • Aldehydes and unsymmetrical ketones give rise to optical isomers
  • A racemic mixture is produced
  • This is a 50:50 mixture of the the two enantiomers
  • It is produced as the planar carbonyl is equally susceptible from attack by the cyanide nucleophile from either side
  • As HCN is a highly toxic gas it is generated in the reaction mixture by adding dilute acid to an excess of sodium cyanide (aq) 
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