Which of the following trihalides of `P`is least acidic?

Options

A. `PI_(3)`
B. `PBr_(3)`
C. `PCl_(3)`
D. `PF_(3)`

Answer

1

Explanation

The trihalides of `P, As` and `Sb` (esecially the fluorides and chlorides ) show acceptor porperties due to the presence of d-orbitals i.e. they can accept an electrons pair and can act as Lewis acids:
`PF_(3)+F_(2)rarrPF_(5)`
`PCl_(3)+Cl_(2)rarrPCl_(3)`
`SbF_(3)+2F^(-)rarr[SbF_(5)]^(2-)`
However, the Lewis acid strength decreases in the order: `PCl_(3) gt AsCl_(3) gt SbCl_(3)` (due to increasing size of `d` orbitals).`PF_(3) gt PCl_(3) gt PBr_(3) gt PI_(3)` (due to increasing siza of halogen).