Question

Why ${H_2}S{O_4}$ cannot be used along with $KI$ in the conversion of an alcohol to an alkyl iodide?

Answer 1

Sulphuric acid also known as oil of vitriol is a mineral acid composed of sulfur, oxygen and hydrogen. It is a colorless, odourless, and viscous liquid that is soluble in water and is synthesized in reactions that are highly exothermic. Due to its affinity towards water, pure anhydrous sulfuric acid does not exist in nature.

Complete step by step answer:
Sulphuric acid is a very strong acid. In aqueous solution, it ionizes completely to form hydronium ions (${H_3}{O^ + }$) and hydrogen sulfate ions ($HS{O_4}^ -$).
Moreover, in addition to being an oxidizing agent, reacting readily at high temperatures with many metals, it is also a strong dehydrating agent and combines violently with water.
But, in the presence of sulfuric acid, KI produces HI. The reaction is given below:
$2KI + H{}_2S{O_4} \to 2KHS{O_4} + 2HI$
Since, ${H_2}S{O_4}$ is an oxidizing agent, it oxidizes with $HI$.
$2HI + {H_2}S{O_4} \to {I_2} + S{O_2} + {H_2}O$
Therefore, the reaction between alcohol and HI to produce alkyl iodide cannot occur and hence, sulphuric acid can’t be used.

Note: The primary application of sulphuric acid is in fertilizer processing e.g. lime superphosphate and ammonium sulfate. It is commonly used in chemical processing e.g. in the manufacture of hydrochloric acid, nitric acid, sulfate salts, synthetic detergents, dyes and pigments, explosives and medicines.