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

From hydrocarbons

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

Rosenmund's reduction

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.

From nitriles

1. 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 Stephen's reduction.
   
   

Imines

Imines contain carbon-nitrogen double bond and have the following structure :


2. Nitriles can alternatively be reduced by diisobutylaluminium hydride (DiBAL-H) to imines, which on hydrolysis gives corresponding aldehydes.
   
3. Similarly, esters are also reduced to aldehydes with (DiBAL-H).
   
4. Treatment of nitriles with Grignard's reagents followed by hydrolysis yield ketones.
   
   

   Exception : HCN reacts with Grignard's reagents to form aldehyde (not ketone).