5.Preparation of Haloarenes

(i) From hydrocarbons by electrophilic substitution

• Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride.

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• The ortho and para isomers can be easily separated due to large difference in their melting points.

• Reactions with iodine are reversible in nature and require the presence of an oxidising agent (HNO3 , HIO4 ) to oxidise the HI formed during iodination.

• Fluoro compounds are not prepared by this method due to high reactivity of fluorine.

(ii) From amines by Sandmeyer’s reaction

• When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed.

• Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by –Cl or –Br.

• Replacement of the diazonium group by iodine does not require the presence of cuprous halide and is done simply by shaking the diazonium salt with potassium iodide.