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.
- 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
- 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+
Reducing aldehydes and ketones
- 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
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)