Reactions of some phosphorus(III) halides with strongly acidic solvents

Abstract

The reactions of PX₃(X = Cl, Br, or I) with the strongly acidic solvents 100% H₂SO₄, 25 and 65 oleum, HSClO₃ and HSFO₃, have been investigated by means of ³¹P n.m.r. spectroscopy. The usual initial reaction is oxidation to [PX₃(OH)]⁺, although HSClO₃ also forms [PX₃Cl]⁺. The reduction in acid strength which necessarily accompanies the redox reaction promotes solvolysis of P–X bonds, and halogen exchange in HSClO₃ and HSFO₃. Formation of [PBr₄]⁺ from PBr₃has also been observed in several solvents. Some interesting new species, notably phosphorus(V) cations containing P–I bonds, have been identified in solution, but not in 25 oleum. Condensation reactions involving elimination of HBr or HI may occur in 65 oleum. Possible mechanisms for the various reactions observed are proposed.