Phosphoryl Tribromide, POBr₃

Phosphoryl Tribromide, POBr₃, is produced by reactions analogous to those which are used in the preparation of phosphorus oxychloride—(a) By the gradual hydrolysis of the pentabromide. (b) By the oxidation of the tribromide at its boiling-point with oxygen, or in the cold with oxides of nitrogen. (c) By the mutual action between P₂O₅ and PBr₅ according to the equation

P₂O₅ + 3PBr₅ = 5POBr₃

(d) By the oxidation of P₄O₆ with Br₂, thus

P₄O₆ + 4Br₂ = 2POBr₃ + 2PO₂Br

Phosphoryl bromide is a solid which crystallises in colourless plates. The formula was established by analysis and vapour density. The molecular weight as determined by the cryoscopic method is 300 in benzene. The density of the solid is given as 2.822. The melting-point is given as 45° c., 56° c. The boiling-point is 193° c. (758 mm.) or 198° c. (760 mm.). As would be expected, the heat of formation from its elements is considerably less than that of POCl₃, and is given by the equation

P (solid) + ½O₂ + 1½ Br₂ (gas) = POBr₃ (solid) + 120 Cals.
P (solid) + ½O₂ + 1½Br₂ (liquid) = POBr₃ (solid) + 108 Cals.

The heat of oxidation of PBr₃ was found to be slightly less than that of PCl₃, and is given by the equation

PBr₃ + 1½O₂ = P0Br₃ (solid) + 64.88 Cals.

Hydrolysis follows the usual course, and the heat evolved per mol of POBr₃ hydrolysed was 75 Cals.