Sulphur Oxytetrachloride, S₂O₃Cl₂

During the preparation of sulphur monochloride, Millon in 1838 obtained a substance which he described as a crystalline sulphur chloride; its formation was due to the presence of moisture during the action, and similar crystals can be formed by introducing a little sulphur monochloride into a flask filled with moist chlorine. The composition of the crystals has since been shown to be represented by Sulphur Oxytetrachloride, S₂O₃Cl₂.

A more convenient method of preparation is to pass chlorine into a mixture of sulphur monochloride with a bimolecular proportion of chlorosulphonic acid at -15° C., until solidification occurs. Probably sulphur tetrachloride is first formed and then reacts with the chlorosulphonic acid:

SCl₄ + Cl.SO₂.OH = Cl.SO₂.O.SCl₈ + HCl

Sulphur oxytetrachloride is a pungent, colourless, crystalline solid of m.pt. 57° C., with decomposition. Even when kept at the ordinary temperature it gradually liquefies on account of decomposition into thionyl and sulphuryl chlorides; if decomposed rapidly at higher temperatures, the latter compound is naturally accompanied by or even replaced by its dissociation products.

S₂O₈Cl₂ = SOCl₂ + SO₂Cl₂.

Treatment with water causes hydrolysis to sulphuric and sulphurous acids, together with hydrochloric acid:

S₂O₃Cl₂ + 4H₂O = H₂SO₃ + H₂SO₄ + 4HCl.

Sulphuric acid reacts with sulphur oxytetrachloride as with sulphuryl chloride, yielding chlorosulphonic acid.

In accordance with its reactions the constitution of sulphur oxytetrachloride must be regarded as represented by Cl.SO₂.O.SCl₃.