JEE Main · 2022 · 26 Jul 2022 · Shift 2 · p-Block Elements

First reaction: Ethanol reacts with $\mathrm{PCl}{}_3$ to give ethyl chloride and phosphorous acid:
$\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}+\mathrm{PCl}{}_3\to \mathrm{C}{}_2\mathrm{H}{}_5\mathrm{Cl}+\mathrm{H}{}_3\mathrm{PO}{}_3$ (A = $\mathrm{H}{}_3\mathrm{PO}{}_3$)

Second reaction: Phosphorous acid reacts with $\mathrm{PCl}{}_3$ to form pyrophosphorous acid:
$\mathrm{H}{}_3\mathrm{PO}{}_3+\mathrm{PCl}{}_3\to \mathrm{H}{}_4\mathrm{P}{}_2\mathrm{O}{}_5+\mathrm{HCl}$ (B = $\mathrm{H}{}_4\mathrm{P}{}_2\mathrm{O}{}_5$)

In pyrophosphorous acid ($\mathrm{H}{}_4\mathrm{P}{}_2\mathrm{O}{}_5$), the structure shows two phosphorus atoms bridged by oxygen, with each P having one P-H bond and P-OH groups. The two P-H protons are non-ionisable (directly bonded to phosphorus), while the two O-H protons are ionisable. Therefore, the number of non-ionisable protons = 2.