Aryl chlorides and bromides can be easily prepared by electropilic 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?

Explanation

Iodination of hydrocarbon (RH or ArH) is a reversible reaction. To shift the equilibrium to the forward direction, HI formed during the reaction should be removed by using an oxidising agent, e.g., `HNO_(3) or HIO_(3)`

`2HI+2HNO_(3) = I_(2)+2NO_(2)+2H_(2)O`
`5HI+HIO_(3)=3I_(2)+3H_(2)O`