AQA Chem 4: Carbonyl Compounds, Aldehydes and Ketones

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.  

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+

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

• 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)