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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.

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