(i) Preparation of Aldehyde
1. From the reduction of alkyl nitrile (Stephen Reduction):
When the solution of alkyl nitrile in ether is reduced with stannous chloride in the presence of hydrochloric acid at room temperature then Imine hydrochloride precipitated, which on hydrolysis with hot water gives aldehyde.
This reaction is called Stephen Reduction.
2. From acyl chloride (acid chlorides) by Rosenmund Reduction:
Acid chloride, when reduced with hydrogen in boiling xylene in presence of palladium catalyst (BaSO4 + S or quinolone), gives aldehyde.
This reaction is called Rosenmund’s reduction. Formaldehyde cannot be prepared by this method because HCOCl is unstable at room temperature. Generally, aldehydes on further reduction give primary alcohols but here BaSO4 and sulphur or quinoline act as catalytic poison for Pd catalyst and prevent the reduction of aldehyde to alcohol.
(ii) Methods for Preparation of Ketones
1. From acid chlorides or Acyl Chlorides:
The ketones are easily prepared by reaction of suitable dialkyl cadmium with acid chlorides.
In this reaction, the Grignard reagent cannot be used directly because it is more reactive than dialkyl cadmium and converts the ketone produced in reaction into tertiary alcohols.
2. From Nitriles:
Ketones can be easily prepared by the reaction of the Grignard reagent with suitable alkyl nitrile.
In the above reaction, if hydrocyanic acid is used in place of alkyl nitrile then aldehydes are formed.
3. From ketonic hydrolysis of Acetoacetic ester or its alkyl derivative:
Acetoacetic ester or its alkyl derivatives are heated in the presence of dilute bases gives ketones after ketonic hydrolysis.
