Sulphur Dichloride, SCl₂

As explained already, the existence of Sulphur Dichloride, SCl₂, has been the subject of much controversy, the view held by some chemists being that the liquid described by others as sulphur dichloride is in reality a mixture of the monochloride with the tetrachloride or with chlorine. A difficulty encountered in the characterisation of such a substance is that molecular weight determinations with the gaseous or dissolved substance do not distinguish between SCl₂ molecules and a mixture of S₂Cl₂ and Cl₂ molecules in equal numbers.

The freezing-point curves for mixtures of sulphur monochloride and chlorine containing up to 92 per cent, of total chlorine, as obtained by earlier workers, supplied no evidence of the existence of sulphur dichloride, the only maxima on the curve being found at -80° C. for a composition corresponding with sulphur monochloride, S₂Cl₂, and at -30.5° C. for the composition of sulphur tetrachloride, SCl₄; some indication of a more highly chlorinated derivative such as SCl₁₁ was observable, but there was no suggestion of the existence of SCl₂. This result, of course, did not exclude the possibility of the existence of a compound SCl₂, or its formation from sulphur monochloride and sulphur at higher temperatures.

Beckmann in 1906, however, by the artifice of employing liquid chlorine as an ebullioscopic solvent, found indication of the existence of SCI 2 molecules; with this solvent a mixture of S₂Cl₂ and Cl₂ molecules will not produce the same effect as simple SCl₂ molecules.

In 1927, Lowry succeeded in freezing out some sulphur dichloride from an over-chlorinated equilibrium mixture having its composition adjusted to that of SCl₂ by addition of the monochloride. The product was recrystallised from light petroleum, and upon analysis yielded the empirical formula SCl₂.

An equilibrium mixture of composition corresponding to SCl₂ is most easily prepared by allowing a solution of sulphur monochloride in liquid chlorine to warm to the ordinary temperature; a modification of this process is to saturate the monochloride, cooled in a freezing mixture, with chlorine and subsequently remove excess of the latter gas by a stream of carbon dioxide. Powdered absorbent charcoal acts catalytically in facilitating the action. As prepared in this way, the mixture is a deep, reddish-brown liquid, of density 1.622 at 15° C. It boils at 59° C. under atmospheric pressure, and at -24° C. under 4 mm., but on account of the very considerable dissociation into monochloride and chlorine, the boiling-point is not constant. When cooled, it solidifies near -88° C. and remelts at -78° C.

Sulphur dichloride is decomposed by water, giving hydrochloric and thiosulphuric acids, the latter gradually yielding sulphurous acid and sulphur. This recalls the corresponding behaviour of the monochloride, and, probably on account of the considerable dissociation, even in the liquid condition, the chemical properties of the "dichloride" are generally similar to those of the monochloride.