Reaction of thionyl chloride with white phosphorus forms a… — JEE Main 2023 Chemistry PYQ

JEE Main 2023 · 25 Jan · Shift 1

Reaction of thionyl chloride with white phosphorus forms a compound [A], which on hydrolysis gives [B], a dibasic acid. [A] and [B] are respectively

A.
$\text{PCl}_3$ and $\text{H}_3\text{PO}_3$
B.
$\text{P}_4\text{O}_6$ and $\text{H}_3\text{PO}_3$
C.
$\text{PCl}_5$ and $\text{H}_3\text{PO}_4$
D.
$\text{POCl}_3$ and $\text{H}_3\text{PO}_4$

Show answer and solution

Correct option: A

Solution

White phosphorus (P₄) reacts with SOCl₂ (thionyl chloride) to form PCl₃ (compound A), with SO₂ as the byproduct: P₄ + 6SOCl₂ → 4PCl₃ + 6SO₂. PCl₃ on hydrolysis gives H₃PO₃ (phosphorous acid, compound B): PCl₃ + 3H₂O → H₃PO₃ + 3HCl. H₃PO₃ is dibasic (not tribasic) because one H is directly bonded to P and not ionisable, consistent with the clue that B is a dibasic acid.

[JEE Main 2023 · 25 Jan · Shift 1]

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