Sulphur Monobromide, S₂Br₂

Sulphur Monobromide, or Disulphur Dibromide, S₂Br₂, is the only definite bromide of sulphur. It is obtained on heating an equi-atomic mixture of sulphur and bromine at 100° C. in a sealed tube. The product is distilled under reduced pressure and collected in a dry receiver. Combination of the two elements can also occur at a lower temperature, as is shown by the fact that sulphur dissolved in bromine is present as sulphur monobromide molecules and not as free sulphur, proof being possible by cryoscopic measurements.

Sulphur monobromide is also formed when sulphur monochloride and potassium bromide are heated together in a sealed tube. It is a garnet-red, oily liquid, which does not wet glass; its density is 2.6355 at 20° C.; its melting-point is -46° C. and its boiling-point 54° C. at 0.18 mm.; at higher pressures the boiling-point is inconstant, due to dissociation. The refractive index, n₇₈₂₀, is 2.6268.

Sulphur monobromide is a slightly exothermic substance with respect to its elements. It is decomposed by water in an analogous manner to the monochloride, but more readily. It dissolves sulphur readily on warming, depositing some of the solid again on cooling. Yellow phosphorus dissolves in the cold liquid, but on warming explosive chemical action sets in. Iodine monochloride reacts giving iodine bromide and sulphur monochloride.

In the presence of alkali the bromide decomposes with formation of a sulphite, a bromide and sulphur:

2S₂Br₂ + 6KOH = 4KBr + K₂SO₃ + 3S + 3H₂O.

At one time the existence of compounds SBr₂ and SBr₄ was suspected, but the evidence is insufficient and unconvincing. The curves representing the vapour pressures and the freezing-points of mixtures of sulphur monobromide and bromine give no indication of the formation of any compound. The evidence thus supplied merely indicates that neither dibromide nor tetrabromide is formed on mixing the monobromide and bromine.