Question Details

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

Options

$[P F_{4}]^{+} [P F_{6}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$

$[P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$

$P F_{3}$ and $P C l_{3}$

$P F_{5}$ and $P C l_{3}$

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Correct Answer :

Option B

$[P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$

[PCl4]+[PCl4F2]– and [PCl4]+[PF6]–

Solution :

The correct answer is:

$[P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$

Explanation and Step-by-Step Derivation:

1. Nature of Phosphorus Pentachloride:
Phosphorus pentachloride ( $P C l_{5}$ ) is known to exist as an ionic solid in its crystalline state, composed of $[P C l_{4}]^{+}$ and $[P C l_{6}]^{-}$ ions. In polar organic solvents, ionizing processes and substitution/fluorination reactions readily occur to yield related ionic species.

2. Fluorination Reaction in Polar Solvents:
When $P C l_{5}$ undergoes reaction with fluorinating agents (such as arsenic trifluoride, $A s F_{3}$ ) in a polar organic solvent, halide exchange takes place.

3. Formation of Ionic Adducts:
- During the initial/partial fluorination stages, some chlorine atoms in the anion are replaced by fluorine, yielding the mixed chlorofluorophosphate species:
$2 P C l_{5} + fluorinating agent \to [P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$
- As fluorination proceeds further, complete halogen exchange at the anionic center occurs to form the thermodynamically stable hexafluorophosphate anion, while the cation remains as the tetrachlorophosphonium ion:
$[P C l_{4}]^{+} [P F_{6}]^{-}$

Hence, the species formed on fluorination of phosphorus pentachloride in a polar organic solvent are indeed $[P C l_{4}]^{+} [P C l_{4} F_{2}]^{-}$ and $[P C l_{4}]^{+} [P F_{6}]^{-}$ .