Ncert Exercises & Solutions

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. But why does preparation of aryl

iodides requires presence of an oxidising agent?

lodination reactions are reversible in nature.

$C_{6} H_{6} I_{2} ⇌ C_{6} H_{5} I + H I$

On seeing above reaction it has been observed that hydrogen iodide is formed apart from the required product. The product removal is necessary from reaction mixture in order to prevent the backward reaction,

For carrieng out the reaction in the forward direction, HI which is formed during this reaction is removed through oxidation using oxidising agent. Such as HIO4 or HNO3. The reaction will be as follows

$5 H I + H I O_{3} \to 3 I_{2} + 3 H_{2} O;$
$2 H I + 2 H N O_{3} \underset{}{\to} I 2 + 2 N O_{2} + 2 H_{2} O$
With the use of a suitable oxidising agent, this will be oxidised to give $l_{2}$

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