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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?
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Verified Answer
Preparation of aryl iodide by electrophilic substitution of arene with iodine is a reversible reaction.
$$
\mathrm{C}_6 \mathrm{H}_6+\mathrm{I}_2 \rightleftharpoons \mathrm{C}_6 \mathrm{H}_5 \mathrm{I}+\mathrm{HI}
$$
$\mathrm{HI}$ formed in the above reaction has to be removed from the reaction mixture in order to prevent backward reaction. This $\mathrm{HI}$ is removed by using oxidising agent such as $\mathrm{HNO}_3$ or $\mathrm{HIO}_3$ which oxidise $\mathrm{HI}$ to $\mathrm{I}_2$
$$
\begin{aligned}
&5 \mathrm{HI}+\mathrm{HIO}_3 \longrightarrow 3 \mathrm{I}_2+3 \mathrm{H}_2 \mathrm{O} \\
&2 \mathrm{HI}+2 \mathrm{HNO}_3 \longrightarrow \mathrm{I}_2+2 \mathrm{NO}_2+2 \mathrm{H}_2 \mathrm{O}
\end{aligned}
$$
$$
\mathrm{C}_6 \mathrm{H}_6+\mathrm{I}_2 \rightleftharpoons \mathrm{C}_6 \mathrm{H}_5 \mathrm{I}+\mathrm{HI}
$$
$\mathrm{HI}$ formed in the above reaction has to be removed from the reaction mixture in order to prevent backward reaction. This $\mathrm{HI}$ is removed by using oxidising agent such as $\mathrm{HNO}_3$ or $\mathrm{HIO}_3$ which oxidise $\mathrm{HI}$ to $\mathrm{I}_2$
$$
\begin{aligned}
&5 \mathrm{HI}+\mathrm{HIO}_3 \longrightarrow 3 \mathrm{I}_2+3 \mathrm{H}_2 \mathrm{O} \\
&2 \mathrm{HI}+2 \mathrm{HNO}_3 \longrightarrow \mathrm{I}_2+2 \mathrm{NO}_2+2 \mathrm{H}_2 \mathrm{O}
\end{aligned}
$$
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