Question
Why does the iodination of methane require an oxidising agent while so much reagent is needed in the chlorination and bromination of methane?
Solution
Iodine reacts with methane molecules reversibly. In fact, hydrogen iodide formed is a very strong reducing agent and it can convert iodomethane back to methane.
To overcome this difficulty, iodination is generally carried out in the presence of strong oxidising agents like iodic acid (HIO3) which oxides HI formed during the reaction to iodine. 
On the other hand, during the chlorination or bromination of methane, no such reducing agent is produced.
