Question

Why is phosphoric acid used instead of sulfuric acid in iodination of alcohols?

Answer 1

Hi there

In the presence of sulphuric acid (H2SO4), KI produces HI

2KI + H2SO4 → 2KHSO4 + 2HI

Since H2SO4 is an oxidizing agent, it oxidizes HI (produced in the reaction to I2).

2HI + H2SO4 → I2 + SO2 + H2O

As a result, the reaction between alcohol and HI to produce alkyl iodide cannot occur. Therefore, sulphuric acid is not used during the reaction of alcohols with KI. Instead, a non-oxidizing acid such as H3PO4 is used.


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