The iodination of benzene is carried out in the presence of an oxidising agent such as HNO3 or HIO4. What is the role of the oxidising agent?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It oxidises the HI formed, so that the reversible reaction goes forward.
Chlorine and bromine substitute an arene easily in the presence of a Lewis acid. The reaction with iodine, however, is reversible: the HI formed can react with iodobenzene and give back benzene and iodine.
\( \displaystyle \ce{C6H6 + I2 <=> C6H5I + HI} \)
The oxidising agent (HNO3 or HIO4) oxidises the HI as it forms. With a product removed, the equilibrium shifts to the right, and iodobenzene is obtained.
Why the other options are incorrect
Option (1) It acts as a Lewis acid catalyst, as iron(III) chloride does in chlorination. — Incorrect. A Lewis acid such as iron(III) chloride is the catalyst for chlorination and bromination. HNO3 and HIO4 are added for their oxidising action, not as Lewis acids.
Option (2) It removes the water formed, so that the reversible reaction goes forward. — Incorrect. The iodination of benzene gives iodobenzene and HI; no water is formed in it. The product that drives the reaction back is HI, so it is HI that has to be removed.
Option (3) It nitrates the benzene ring before the iodine atom enters. — Incorrect. The product is iodobenzene, which has no nitro group. The acid is used here to remove HI, not to put a nitro group on the ring.
NCERT (2026-27 edition):
6.5 Preparation of Haloarenes – “…Reactions with iodine are reversible in nature and require the presence of an oxidising agent (HNO3, HIO4) to oxidise the HI formed during iodination.”