JEE Advanced 2024 — Paper 2 — CHEM-I solution

JEE Advanced2024Paper 2CHEM-I

The species formed on fluorination of phosphorus pentachloride in a polar organic solvent are

A
$[P F_{4}]^{+} [P F_{6}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$
B
$[P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$
C
$P F_{3}$ and $P C l_{3}$
D
$P F_{5}$ and $P C l_{3}$

Solution

$P C l_{5}$ in the solid state itself exists as an ionic pair $[P C l_{4}]^{+} [P C l_{6}]^{-}$ because polar environments stabilise the ionic form over the molecular trigonal bipyramidal form.

When $P C l_{5}$ is partially fluorinated in a polar organic solvent, fluoride preferentially replaces chloride on the anion (octahedral coordination tolerates the smaller F more readily). Two ionic isomers are obtained:

1. $[P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$ — a colourless crystalline solid where partial fluorination of the anion has occurred.

2. $[P C l_{4}]^{+} [P F_{6}]^{-}$ — a white crystalline solid where fluorination of the anion is complete.

The cation $[P C l_{4}]^{+}$ is retained in both because $s p^{3}$ tetrahedral $P C l_{4}^{+}$ is sterically favoured over $P F_{4}^{+}$ .

← Back to full paper

Practice more CHEM-I

Concept-wise practice with instant solutions on Fundamenthol.

Start practicing →