Sulphur Tetrachloride, SCl₄

When a mixture of sulphur monochloride with liquid chlorine is kept in a sealed tube, slow combination ensues, with formation of Sulphur Tetrachloride, SCl₄. At ordinary temperatures the rate of combination is very slow, but it is hastened by a moderate rise in temperature.

The product is a brownish-red liquid at ordinary temperatures; when cooled it exhibits in a marked manner the phenomenon of "suspended transformation," and generally does not solidify above -70° C., the yellowish-white solid obtained below this temperature melting at -30.5° C. A few degrees above the melting-point the dissociation pressure reaches one atmosphere, so that dissociation into sulphur monochloride and chlorine, probably with the production of sulphur dichloride as an intermediate stage, occurs exceedingly readily, and at the ordinary temperature the liquid contains only a small proportion of the tetrachloride, the dissociation products preponderating (see the following).

Water causes immediate and almost quantitative decomposition of sulphur tetrachloride, with formation of hydrochloric and sulphurous acids, the latter standing in the same relation to sulphur tetrachloride as carbonic acid to carbon tetrachloride and silicic acid to silicon tetrachloride:

SCl₄ + 3H₂O = H₂SO₃ + 4HCl.

With anhydrous ammonia the tetrachloride reacts to form nitrogen sulphide:

12SCl₄ + 16NH₃ = 3N₄S₄ + 48HCl + 2N₂.

With an equimolecular proportion of sulphur trioxide, interaction occurs forming thionyl chloride, sulphur dioxide and chlorine:

SCl₄ + SO₃ = SOCl₂ + SO₂ + Cl₂,

but an excess of sulphur trioxide leads to simultaneous formation of pyrosulphuryl chloride:

SCl₄ + 2SO₃ = S0Cl₂ + S₂O₅Cl₂. Sulphur dioxide does not exert any action on sulphur tetrachloride, but above 0° C. chlorosulphonic acid yields the same products as those given in the first equation for the action of sulphur trioxide, whilst below 0° C. the product is sulphur oxytetrachloride, S₂O₃Cl₄.

Many chlorides, especially those of the metals, combine with sulphur tetrachloride to produce unstable crystalline additive compounds; thus, iodine trichloride, antimony pentachloride, titanium tetrachloride, stannic chloride, ferric chloride, and also arsenic fluoride, yield crystalline products containing the added molecule SCl₄; this provides strong evidence of the definite existence of this chloride of sulphur.

In its action on organo-magnesium compounds, sulphur tetrachloride behaves as if it consists only of sulphur dichloride and chlorine, and its reactions with sulphur trioxide (see before) appear to be capable of a similar interpretation; in these cases it is indeed possible that sulphur dichloride and chlorine, present as products of dissociation, are the actual agents in the chemical change.