General Methods Of Preparation Of Aliphatic Aldehydes And Ketones

1. From Alcohols

(a) By Direct oxidation: Aldehydes are prepared by oxidation of 1° alcohols. Refer to Alcohols

(b) By catalytic dehydrogenation

When vapours of 1° or 2° alcohols are passed over copper gauze, they get dehydrogenated to form aldehydes or ketones.

Dehydrogenation reaction is a better method of preparation because there is no risk of further oxidation of aldehyde.

(c) By using PCC

PCC stands for pyridinium chlorochromate. It is an equimolar mixture of CrO₃, HCl and pyridine. It is used to oxidise 1° alcohol to aldehyde and 2° alcohol to ketones.

2. From Acid chlorides

Aldehydes are prepared from acid chlorides by reaction with H2 in the presence of palladium catalyst supported on BaSO4.

(Rosenmund’s reduction)

Ketones are obtained by reacting acid chlorides with dialkyl cadmium.

3. From fatty acids

(a) By heating calcium salt of fatty acid

Aldehydes are obtained by heating calcium salt of fatty acids with calcium formate.

Ketones are formed by distilling calcium salt of fatty acid alone.

Similarly mixed ketones, can also be obtained by similar reactions:

(b) By passing vapours of fatty acids over manganese oxide

In this method, formic acid alone gives formaldehyde. Acetic acid gives acetone & the mixture of two acids gives acetaldehyde.

4. From Alkynes: (Refer to hydrocarbons)

5. By reductive ozonolysis of alkenes: (Refer to hydrocarbons)