Saturday, January 26, 2008

IIT JEE Revision Benzene Preparation

Preparation of benzene and other aromatic compounds or arenes

1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.

3C2H2 gives C6H6

Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.

2. From aryl halides:
Benzene is obtained from chlorobenzene by reducing it with Ni-Al alloy in the presence of sodium hydroxide

Arenes are obtained by reaction fo aryl halide, sodium metal and alkyl halide in dry ether.

Bromobenzene + sodium + Ethylbromide give Ethylbenzene and Sodiumbromide

3. Arenes from Benzene and alkyl halides
Arenes can also be obtained from benzene and alkyl halides in the presence of anhydrous aluminium chloride. This is called Friedel Craft's reaction.

4.From Grignard reagent (Phenyl magnesium halide)
Arenes are also prepared by reacting aromatic Grignard reagent and alkyl halide.

5. From phenol: by distillation of phenol with zinc.

6 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime. Decarboxylation: Removal carboxyl group

7. From diazonium salts: Benzene diazonium is reduced by hypophosphorus acid.

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