KI is expected to give HI on reacting with `H_(2)SO_(4)` which will convert alcohols `(R-OH)` to alkyl iodides (R-I). Howerver, `H_(2)SO_(4)` is a strong oxidising agent and it oxidises HI formed during the reaction to `I_(2)` which does not reacts with alcohol.
`KI+H_(2)SO_(4)overset(Heat)toKHSO_(4)+HI`
`H_(2)SO_(4)toH_(2)O+SO_(2)+(O)`
`2HI+(O)toH_(2)O+I_(2)`
To solve the problem, `H_(2)SO_(4)` is replaced by phosphric acid `(H_(3)PO_(4))` which provides HI for the reaction and does not give `I_(2)` as is done by `H_(2)SO_(4)`.
`KI+H_(3)PO_(4)tounderset("pot. dihydrogen phosphate")(KH_(2)PO_(4)+HI)`
`underset("Alcohol")(R-OH+)HItounderset("Alkyl iodide")(R-I+H_(2)O)`
