Pyrosulphuryl Chloride, S₂O₅Cl₂

Rose first prepared Pyrosulphuryl Chloride, S₂O₅Cl₂, in 1838 by the interaction of sulphur trioxide and sulphur monochloride. It is formed by the action of many chlorides on sulphur trioxide; sulphur monochloride, thionyl chloride, silicon tetrachloride, phosphorus pentachloride, phosphorus oxychloride, sodium chloride and carbon tetrachloride all yield the desired product when treated with sulphur trioxide at a suitable temperature. With sodium chloride a mixture of pyrosulphuryl chloride and sodium pyrosulphate is obtained, whilst with the exception of carbon tetrachloride and phosphorus pentachloride, which are converted into oxychlorides, all the remaining chlorides are changed into oxides:

5SO₃ + S₂Cl₂ = S₂O₅Cl₂ + 5SO₂;
2SO₃ + SOCl₂ = S₂O₅Cl₂ + SO₂;
2SO₃ + PCl₅ = S₂O₅Cl₂ + POCl₃;
6SO₃ + 2P0Cl₃ = 3S₂O₅Cl₂ + P₂O₅;
2SO₃ + 2SiCl₄ = S₂O₅Cl₂ + Si₂OCl₆,
followed by 6SO₃ + Si₂OCl₆ = 3S₂O₅Cl₂ + 2SiO₂;
4SO₃ + 2NaCl = S₂O₅Cl₂ + Na₂S₂O₇;
2SO₃ + CCl₄ = S₂O₅Cl₂ + COCl₂.

The most convenient process is to add a mixture of sulphur trioxide and fuming sulphuric acid gradually to hot carbon tetrachloride, the product being purified by fractional distillation, followed by treatment with sodium chloride to remove any chlorosulphonic acid and subsequent redistillation, preferably under reduced pressure. Fuming sulphuric acid reacts with carbon tetrachloride at 150° C. with formation of chlorosulphonic acid and carbonyl chloride. The chlorosulphonic acid then reacts with more carbon tetrachloride thus:

CCl₄ + 2SO₃HCl = S₂O₅Cl₂ + 2HCl + COCl₂.

When chlorosulphonic acid is dehydrated by the action of phosphorus pentoxide or pentachloride, pyrosulphuryl chloride is obtained:

Cl.SO₂.O = Cl.SO₂.O.SO₂Cl + H₂O.

Pyrosulphuryl chloride is a colourless, rather viscous liquid, which in the air fumes less strongly than sulphur trioxide. It boils at 152° to 152.5° C. under 766 mm. pressure, at 57° C. under 30 mm., and at 52° C. under 15 mm., without appreciable decomposition if dry.

D⁰₄ = 1.872, D²⁰₄ = 1.837; m.pt. = -37.5° to -37° C.; n¹⁹_D = 1.449. From vapour density determinations it is known that at 180° C. the molecular weight is normal but that at higher temperatures decomposition occurs. The liquid chloride is exothermic with respect to its elements, the heat of formation, according to Ogier, being 159,400 calories, but according to Konowalov 188,200 calories. The specific heat of the liquid is 0.258, and the molecular heat of vaporisation according to the last-named investigator is 7550 calories, a figure more in accordance with Trouton's Rule than the high value of 13,160 calories given by Ogier.

When passed through a red-hot tube the vapour is almost entirely decomposed into sulphur dioxide, sulphur trioxide and chlorine. Vapour density determinations at various temperatures, and analyses of the decomposition products, indicate that decomposition proceeds according to the equations:

S₂O₅Cl₂ = SO₃ + SO₂ + Cl₂,
and S₂O₅Cl₂ = SO₃ + SO₂Cl₂,

the former reaction occurring to a small extent at 200° C. in the presence of sulphur dioxide and chlorine, but giving place at higher temperatures to the second reaction, when

SO₂Cl₂ SO₂ + CI₂

follows and decomposition is complete at 360° C.

The liquid chloride is only gradually attacked by water, giving sulphuric and hydrochloric acids, but if the relative quantity of water is very small, two reactions occur,

S₂O₅Cl₂ + H₂O = 2HClO + 2SO₂,
SO₂ + 2HClO = H₂SO₄ + Cl₂,

such solutions having oxidising properties.

Like sulphuryl chloride, pyrosulphuryl chloride can convert many elements, e.g. sulphur, phosphorus, antimony and mercury, into the corresponding chlorides, with simultaneous formation of sulphur dioxide and trioxide. In the reaction between pyrosulphuryl chloride and phosphorus pentachloride or trichloride, there are obtained phosphorus oxychloride, sulphur dioxide and chlorine.

Pyrosulphuryl chloride is not miscible with sulphuric acid, but when sealed together in a tube the two liquids become homogeneous in the course of a few weeks, the sulphuric acid undergoing dehydration:

S₂O₅Cl₂ + H₂SO₄ = 2Cl.SO₂.OH + SO₃.

Hydrogen bromide, iodide and sulphide are oxidised to the corresponding elements, various reduction and hydrolytic products of the pyrosulphuryl chloride being formed at the same time. Phosphine also causes reduction of pyrosulphuryl chloride with production of sulphide of phosphorus.

With chromates, chromyl chloride is produced:

K₂CrO₄ + S₂O₅Cl₂ = CrO₂Cl₂ + K₂S₂O₇.

Selenium is oxidised to a selenium sulphur oxytetrachloride.

From the chemical behaviour of the substance and especially its relationship to chlorosulphonic acid, it is evident that pyrosulphuryl chloride may be regarded as chlorosulphonic anhydride, Cl.SO₂.0.SO₂.Cl. Grignard and Muret consider that the properties of the compound are best accounted for by the formula

S(:O)(OCl).O.S(:O)(OCl).