Question

How are the following preparations carried out?

(1) Benzaldehyde from toluene. (Etard oxidation)

(2) Benzaldehyde from methyl arene.

(3) Benzaldehyde from toluene (commerical method).

(4) Benzaldehyde from benzene (GattermannKoch synthesis).

OR 

Write chemical equation for Gatter- mannKoch formylation.

Answer 1

(1) Benzaldehyde from toluene. (Etard oxidation) :

When toluene is treated with solution of chromyl chloride (CrO₂Cl₂) in Cs₂, brown chromium complex is obtained, which on acid hydrolysis gives benzaldehyde. 

This reaction is called Etard reaction. 

(2) Benzaldehyde from methyl arene :

Methylarene is converted into a benzyllidene diacetate on treatment with chromium oxide in acetic anhydride at 273-278 K. The diacetate derivative on acid hydrolysis gives benzaldehyde.

(3) Benzaldehyde from toluene (commerical method) :

Side chain chlorination of toluene gives benzal chloride which on hydrolysis gives benzaldehyde.

(4) Benzaldehyde from benzene (Gattermann-Koch synthesis) :

When benzene is treated with vapours of carbon monoxide and hydrogen chloride in the presence of a catalyst mixture of AlCl₃ and CuCl under high pressure, benzaldehyde is obtained. 

This reaction is called Gattermann- Koch synthesis.

Preparation of aromatic ketones from hydrocarbons.