Explain why phosphoric acid and not sulphuric acid is used with KI during the preparation of alkyliodides from alcohols.

Explanation

KI is expected to give HI on reacting with `H_(2)SO_(4)` and it is HI which is expacted to convert alcohols (ROH) to alkyl iodides (R-I). However, `H_(2)SO_(4)` is a strong oxidising agent. It oxidises HI to `I_(2)` which cannot react with alcohols.
`KI + H_(2)SO_(4) overset(Delta)(rarr)KHSO_(4)+2HI`
`2HI + H_(2)SO_(4) rarr H_(2)O + I_(2)+SO_(2)`
To overcome this problem, the non-oxidising acid `H_(3)PO_(4)` is used instead of `H_(2)SO_(4)`
`ROH+KI+H_(3)PO_(4)overset(Delta)(rarr)R-I+KH_(2)PO_(4)+H_(2)O`.