Preparation of Aldehydes and Ketones
By oxidation of alcohols
We have already studied in alcohols and phenols that oxidation of alcohols converts 1° and 2° alcohols to aldehydes and ketones respectively when any one of the following is used :
- PCC (pyridum chlorochromate)
- Collins reagents (Chromium trioxide-pyridine complex)
- Cu at 573 K
Primary alcohols are oxidised to aldehydes
Secondary alcohols are oxidised to ketones
Ozonolysis of alkenes
Alkenes react with O3 in the presence of Zn and H2O to produce aldehydes or ketones depending upon the structure of alkene.
Ozonolysis is discussed in details in reactions of alkenes.
From hydration of alkynes
Alkynes undergo hydration according to Markovnikov's rule in the presence of catalyst HgSO4 and H2SO4 to give ketones.
Exception : Ethyne gives aldehyde instead of ketones.
From acid chlorides
Acid chloride is hydrogenated over catalyst Pd on BaSO4 and partially poisoned by the addition of Sulphur or quinoline to give aldehydes.
Important points regarding Rosenmund's reaction
- Sulphur (or quinoline) acts as poison for catalyst and prevent further reduction into alcohol.
- Formaldehyde cannot be prepared by this method because formyl chloride (HCOCl) is unstable at room temperature.
- Ketones cannot be prepared by this reaction.
Preparation of ketones
Treatment of acid chloride with dialkylcadmium [(R)2Cd] gives ketones.
- Nitriles are first reduced to corresponding imines with SnCl2 in
the presence of HCl, which on hydrolysis give corresponding aldehydes. Preparation of aldehydes by this method is known as
- Nitriles can alternatively be reduced by diisobutylaluminium hydride (DiBAL-H) to imines, which on hydrolysis gives corresponding
- Similarly, esters are also reduced to aldehydes with (DiBAL-H).
- Treatment of nitriles with Grignard's reagents followed by hydrolysis yield ketones.
Exception : HCN reacts with Grignard's reagents to form aldehyde (not ketone).