I am aware that PCl3 and PCl5 both react with water. In these reactions, water breaks the chlorine atoms off of the phosphorus atom and hydroxyl groups take their place:

$\ce{PCl3 + 3H2O -> H3PO3 + 3HCl}$

$\ce{PCl5 + 4H2O -> H3PO4 + 5HCl}$

So, judging by these equations, my prediction is that the tetrachlorophosphonium ($\ce{PCl4+}$) ion should readily react with water as well. This is my guess at how the reaction should go:

$\ce{PCl4+ + 4H2O -> H+ + H3PO4 + 4HCl}$

Is this correct? If so, I predict the tetrahalophosphonium ions (with fluorine, bromine, and iodine) would have the exact same reaction but with HF, HBr, or HI formed instead of HCl.

If this is not correct, what would the resulting hydrolysis products be?

  • $\begingroup$ The reaction would form not only phosphoric acid but also phosphorus acid. Not sure about other halogens. Fluorine will most probably only partially hydrolyze, whereas in case of iodine reducing property of HI might be too high and so you might observe I2 vapors. $\endgroup$ Jan 6, 2022 at 9:19
  • 5
    $\begingroup$ $\ce{PCl4^+}$ reacts like $\ce{PCl5}$, because of the equilibrium $$\ce{PCl5 <=> PCl4^+ + Cl^-}$$ So the reaction with water will produce $\ce{H3PO4 + HCl}$ as stated in the original post, and no phosphorous acid. $\endgroup$
    – Maurice
    Jan 6, 2022 at 9:27


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