AQA Chem 4: Carbonyl Compounds, Aldehydes and Ketones

Revision notes on aldehydes and ketones from AQA Unit 4

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

Intro

 

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.  

1 of 4

Tests

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+
2 of 4

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
3 of 4

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) 
4 of 4

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Functional Groups resources »