# Methods of Preparation of Aldehydes

Methods of Preparation of Aldehydes

The organic compounds that contain carbon-oxygen double bonds are referred to as carbonyl compounds. Carbonyl group is one of the most significant functional group in organic chemistry. Some of these compounds are widely used in the industry for manufacturing various chemicals and reagents.  Carbonyl compounds are of two types, aldehydes and ketones. The compounds in which the carbonyl group is attached to carbon and hydrogen are called aldehydes while the compounds in which the carbon group is attached to two carbon atoms are called ketones. This article is concerned about the methods of preparation of aldehydes.

Preparation of Aldehydes:

There are various methods that can be used to prepare aldehydes depending upon the type and requirement of the compounds. Following are some important methods of preparation of aldehydes.

By oxidation of alcohol:  After the oxidation of primary and secondary alcohols we can get both aldehydes as well as ketones. Oxidation of primary alcohols results in aldehydes while oxidation of secondary alcohols produces ketones

Dehydrogenation of alcohols: This is widely used in industries. In this method primary alcohol is passed over metal catalysts  such that the product obtained is an aldehyde. This method is preferred for the conversion of volatile alcohols into aldehydes.

From hydrocarbons: Aldehydes are obtained by the ozonolysis of alkenes followed by reacting the ozonolysis products with zinc dust and water. A mixture of aldehydes and ketones is obtained depending upon the structure of the hydrocarbon.

There are other methods used for the preparation of aldehydes, let’s have a look on those methods:

• From acyl chloride: The hydrogenation of acid chlorides by passing over over a catalyst such as palladium aldehydes are obtained. This reaction is called Rosenmund reduction.
• From nitriles and esters: We get aldehydes when nitriles are reduced to corresponding imine in the presence of stannous chloride and hydrochloric acid and the resulting mixture is hydrolysed. This reaction is called Stephen reaction.

We can get aromatic aldehydes with the help of aromatic hydrocarbons by using the methods given below:

By oxidation of methylbenzene: Toluene can be oxidised by strong oxidising agents to benzoic acid. The oxidation can be stopped at the aldehyde stage by using those reagents which are suitable to convert the methyl group to an intermediate that cannot be oxidised further.

1. By using chromyl chloride(CrO2Cl2):We get a chromium complex when chromyl chloride oxidises the methyl group, after further hydrolysis benzaldehyde is obtained. This reaction is called Etard reaction
2. By using chromic oxide(CrO3): When we treat chromic acid in acetic anhydride then toluene or substituted toluene is converted to benzylidene diacetate. Benzylidene diacetate on hydrolysis gives the corresponding benzaldehyde.
3. By Gatterman –Koch reaction: The treatment of carbon monoxide and hydrogen chloride with benzene or its derivative in the presence of aluminium chloride gives benzaldehyde or substituted benzaldehyde. This is called Gatterman –Koch reaction.

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