Thionyl Bromide, SOBr₂

Thionyl Bromide, SOBr₂, analogue of thionyl chloride is best prepared by the prolonged action of hydrogen bromide on thionyl chloride:

SOCl₂ + 2HBr = SOBr₂ + 2HCl.

Sodium bromide or aluminium bromide may be used instead of hydrogen bromide.

It is an orange-yellow liquid, density 2.697 at 15° C., b.pt. 45° C. at 22 mm., 68° C. at 40 mm., and 138° C. (with decomposition) at 773 mm. It decomposes slowly at the ordinary temperature, more rapidly on heating, yielding sulphur monobromide, sulphur dioxide and free bromine:

4SOBr₂ = S₂Br₂ + 2SO₂ + 3Br₂.

It is readily hydrolysed by water. It reacts with organic acids to form acid bromides. Mercury decomposes the vapour with formation of sulphur dioxide.

The liquid appears to contain associated molecules, as the following data indicate: molecular weight, 234 to 246; surface tension, 43.71 dynes per cm. at 17° C.; Trouton coefficient, 25.2.

Besson, when preparing thionyl bromide by the action of boiling thionyl chloride on hydrogen bromide or aluminium bromide, claimed to have isolated the intermediate compound, thionyl chlorobromide, SOClBr, a more volatile liquid than thionyl bromide and therefore separated from it by distillation under reduced pressure. The product isolated was a yellow liquid, density 2.31 at 0° C., boiling with slight decomposition at 115° C. Mayes and Partington, however, could obtain no evidence of such an intermediate compound in the product from the bromination of thionyl chloride. The physical properties of the product were similar to those of a mixture of thionyl chloride and thionyl bromide. Moreover, the boiling-points of mixtures of the two compounds rise evenly from that of the chloride to that of the bromide, whilst the freezing-point curve of such mixtures is a smooth mixed crystal curve, giving no indication of the existence of any intermediate compound.

The remaining compounds of bromine and all those of iodine corresponding with thionyl and sulphuryl chlorides and chlorosulphonic acid are either unknown or their existence is very doubtful. There is no reaction between sulphur dioxide and bromine analogous to that by which sulphuryl chloride is formed from sulphur dioxide and chlorine; this is true between 100° and 200° C., both in the dark and under strong illumination, also in the presence of likely catalysts.