Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalyst. But why does preparation of aryl iodides requires presence of an oxidising agent?

Explanation

lodination reactions are reversible in nature.
`C_(6)H_(6)+I_(2)hArrC_(6)H_(5)I+HI`
In this above reaction, hydrogen iodide is formed apart from the required product. It has to be removed from the reaction miture in order to prevent the backward reaction.
To carry out the reaction in the forward direction, HI formed during the reaction is removed by oxidation using oxidising agent. Such as `HIO_(3) or HNO_(3)`. The reaction is as follow
`5HI+HIO_(3)to3I_(2)+3H_(2)O,`
`2HI+2HNO_(3)toI_(2)+2NO_(2)+2H_(2)O`
By using a suitable oxidising agent HI is oxidised to give `l_(2)`