Pyrophosphoryl Chloride, P₂O₂Cl₄

Pyrophosphoryl Chloride, or Diphosphorus trioxytetrachloride, P₂O₂Cl₄ was first prepared by the action of N₂O₃ or N₂O₄ on PCl₃. It is also produced as an intermediate product in the hydrolysis of POCl₃. Thus when POCl₃ is heated with a tenth of its weight of water in a sealed tube at 100° C. the three phosphoric oxychlorides (phosphoryl, pyrophosphoryl and metaphosphoryl) are present and may be separated by distillation in vacuo. Phosphorus pentachloride (2 mols.) and water (3 mols.) also gave this oxychloride when heated to 126° C. under pressure. When separated by fractional distillation under reduced pressure from POCl₃, which is more volatile, and PO₂Cl, which is less volatile, the pyro-compound appeared as a colourless fuming liquid which had a much lower freezing-point and a higher boiling-point than POCl₃. At ordinary pressures it distilled with decomposition. The analysis agreed with the empirical formula given.

The density is about 1.58. The freezing-point is below -50° C., the boiling-point 210° to 215° C.

The liquid distils with some decomposition, which may be expressed by the equation

3P₂O₃Cl₄ ⇔ 4POCl₃ + P₂O₅

The reaction must be reversible since P₂O₃Cl₄ has been obtained by the interaction of the compounds on the right-hand side.

By the action of PCl₅ the ordinary oxychloride is regenerated, thus P₂O₃Cl₄ + PCl₅ = 3POCl₃

PBr₅ reacts in a similar manner with the production of an oxychloro- bromide, thus

P₂O₃Cl₄ + PBr₅ = 2POBrCl₂ + POBr₃

Hydrolysis leads to the same final products as are formed in the hydrolysis of POCl₃. Pyrophosphoryl chloride also acts in a similar way with organic compounds containing the hydroxyl group giving, e.g., ethyl esters of phosphoric and chlorophosphoric acids.

Addition compounds with lime, magnesia and several other basic oxides were formed in organic solvents such as acetone, or ethyl acetate and other esters, and appeared as crystalline substances, associated usually with two molecules of the solvent, e.g. CaO.P₂O₃Cl₄.2(CH₃)₂CO.