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Aldehydes And Ketones: Preparation

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Aldehydes And Ketones: Preparation - Lesson Summary

As aldehydes and ketones contain the same carbonyl group their properties and methods of preparation are quite similar.

Methods for the preparation of aldehydes and ketones:

The process of oxidation of alcohols involves the loss of a- hydrogen, the hydrogen atom attached to the carbon bearing the OH group, in the presence of reagents such as PCC (or) acidified potassium dichromate (or) acidified potassium permanganate.

Carbonyl compounds can be prepared from alcohols is by the dehydrogenation of alcohols. This method is suitable for the oxidation of volatile alcohols. It involves passing vapours of an alcohol over metal catalysts like silver or copper.

Dehydrogenation of primary alcohols yields aldehydes while that of secondary alcohols yields ketones.

Carbonyl compounds can also be prepared from the ozonolysis of alkenes. Ozonolysis involves the cleavage of the double bond by the addition of ozone to give a cyclic product called ozonide, followed by hydrolysis in the presence of zinc dust.

Hydration of alkynes is another method for the preparation of carbonyl compounds. This method involves the addition of water to alkynes in the presence of mercuric sulphate and sulphuric acid to form aldehydes or ketones as the final product.

Carbonyl compounds can also be prepared by the reduction of acid chlorides.

Aldehydes can be prepared by the reduction of acid chlorides with hydrogen in the presence of palladium on barium sulphate in quinoline. This reaction is called Rosenmund's reduction. Aliphatic and aromatic aldehydes can be prepared by this method. But ketones cannot be prepared by this method.

Ketones are prepared by the treatment of acid chlorides with dialkyl cadmium.

Aldehydes can be prepared by the reduction of nitriles to imines followed by hydrolysis.

Ketones can be prepared by treating a nitrile with a Grignard reagent followed by hydrolysis.

Aromatic aldehydes can be prepared by treating benzene (or) its derivatives with carbon monoxide and dry hydrogen chloride gas, in the presence of anhydrous aluminium chloride or cuprous chloride. This reaction is known as the Gatterman-Koch reaction.

Benzaldehyde or substituted Benzaldehyde can also be prepared by the oxidation of methyl benzene with chromyl chloride to a chromium complex followed by hydrolysis. This reaction is known as the Etard reaction.

Benzaldehyde can also be made from toluene. When toluene (or) substituted toluene is treated with chromic oxide in acetic anhydride, benzylidene diacetate is formed. This, on hydrolysis in the presence of an acid yields the corresponding Benzaldehyde

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