Haloarenes and Method of preparation of Haloarenes

Haloarenes

Haloarenes are the halogen derivatives of aromatic hydrocarbons in which halogen is directly attached to the ring. these are obtained by replacing hydrogen by alpha-halogen atom.

Nomenclature.

Monosubstituted haloarenes.

A general method for preparation of Haloarenes.

1. By direct halogenation of aromatic hydrocarbon.

Aromatic hydrocarbon is hydrogenated directly at dark in presence of Lewis acid such as AlCl₃, FeCl₃, FeBr₃ to produce corresponding haloalkanes.

The reaction of an aromatic hydrocarbon with iodine is reversible. Therefore, iodination is carried out in presence of oxidation agent like HNO₃, HIO₃ etc. with accelerate the rate of forward reaction.

2. From diazonium salts.

Diazonium salts. is obtained by adding the ice-cold solution of NaNO₂ slowly on aniline in HCl. This reaction is Known as diazotization.

Preparation of Chlorobenzene

1. From benzene.

When benzene is chlorinated directly with Cl₂ in presence of FeCl₃ at room temperature chlorobenzene is obtained.

2. From Aniline.

To get chlorobenzene from aniline,aniline is first converted into benzene diazonium chloride.

The product thus obtained is then reacted with Cu₂Cl₂ in presence of HCl to get chlorobenzene.

3. From phenol.

For that, phenol is first refluxed with Zn-dust to get benzene.

Then chlorination of benzene is carried out as to get chlorobenzene.

Preparation of chloroand bromobenzene (Sandmeyers reaction).

When benzene diazonium chloride is treated with (Cu₂Cl₂+HCl) and (Cu₂Br₂+HBr)separately. corresponding chlorobenzene and bromobenzene are formed. This reaction is called Sandmeyers reaction.

Preparation of Iodobenzene.

Benzene in presence of I₂ and HNO₃ give IOdobenzene

Iodobenzene is obtained by adding aueousKI solution to benzene diazonium chloride.