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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

Properties

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.

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