Reactions of some phosphorus(V) halides and halide oxides with strongly acidic solvents

Abstract

The reactions of PCl₃O, PBr₃O, [PCl₄][PCl₆], [NEt₄][PCl₆], and [PBr₄] Br with the highly acidic solvents 100% H₂SO₄, HSClO₃, HSFO₃, 25 oleum, and 65 oleum have been investigated by means of ³¹P n.m.r. spectroscopy. The halide oxides undergo protonation at the phosphoryl oxygen, the extent of which depends on the acid strength of the solvent and the solute concentration. Solutions of PCl₃O are stable, as are PBr₃O solutions in the halogenosulphuric acids, but solvolysis of PBr₃O occurs in 100% H₂SO₄ and 25 oleum and condensation in 65 oleum, probably via HBr elimination. The initial products from PCl₅ are [PCl₄]⁺, [PCl₃(OH)]⁺, or both of these, depending on the solvent; further solvolysis is then observed, together with halogen exchange in HSFO₃. Phosphorus(V) bromide forms [PBr₄]⁺ initially, which may similarly undergo further reaction, but is stable in 65 oleum. The difference in behaviour between PCl₅ and PBr₅ is mainly due to the instability of the [PCl₆]^– ion in these solvents. Possible mechanisms for the observed reactions are proposed.