Sulphur Monochloride, S₂Cl₂

Sulphur Monochloride or Disuiphur Dichloride, S₂Cl₂, the initial product of the combination of sulphur and chlorine, was first thoroughly studied in 1810 by Davy and Buchholz, independently.

Sulphur and chlorine interact slowly at the ordinary temperature but much more readily on warming. The customary procedure is to pass dried chlorine into fused sulphur or over dry " flowers of sulphur " until most of the sulphur has disappeared. The resulting monochloride contains considerable amounts of higher sulphur chlorides in solution, but if the mixture is heated for some time under a reflux condenser the pure monochloride can subsequently be distilled over.

Industrially, much sulphur monochloride is obtained as a byproduct in the manufacture of carbon tetrachloride by the action of chlorine on carbon disulphide in the presence of a suitable catalyst, e.g. iodine:

CS₂ + 3Cl₂ = CCl₄ + S₂Cl₂.

Various other processes can be made to yield sulphur monochloride. The distillation of sulphur with stannous chloride or mercuric chloride yields sulphur monochloride and, indeed, may be regarded as a modification of the method first given. The action of phosphorus pentachloride on sulphur or on metallic sulphides and the action of chlorine on metallic sulphides form closely analogous processes, especially in view of the formation of chlorine as a dissociation product from phosphorus pentachloride. With phosphorus pentachloride the phosphorus is found finally as sulphochloride. Of other methods there may be mentioned the interaction of sulphur or of phosphorus sulphide with thionyl chloride, and the action of dry chlorine on a hot or boiling solution of sulphur in sulphur dichloride or on a strongly heated mixture of barium sulphate with coal, coke or hydrocarbons of high carbon content.

The monochloride may be obtained in a highly purified condition by mixing a high-grade commercial sample with 1 per cent, by weight of a mixture of highly absorbent charcoal and sulphur, distilling in glass apparatus and collecting the fraction distilling above 137° C. This fraction, after addition of more charcoal and sulphur, is redistilled in a vacuum under a pressure of 28 mm., pure sulphur monochloride -distilling at 41° C.

Sulphur monochloride is a golden-yellow liquid which fumes in moist air; it has an unpleasant pungent odour and a hot, bitter taste. At 25° C. the density of the liquid is 1.67328; the surface tension, measured at 22° C. by the capillary rise method, is 40.78; the relative viscosity at 18° C. is 1.908, and the specific heat at 22° C. is 0.22 (±2.8 per cent.). The boiling-point is 138° C. and the melting-point -76° to -75° C. The vapour pressure through the temperature range 0° to 138° C. is given in the following table:

°C.	Vapour Pressure in mm. (P.).	°C.	Vapour Pressure in mm. (P.).	°C.	Vapour Pressure in mm. (P.).
0	3.7	50	43.0	100	257.0
10	6.4	59	60.0	110	351.5
20	10.7	70	93.0	120	469.7
31	18.6	80	135.0	130	615.2
40	28.0	90	186.4	138	760.0

The equation for the corresponding curve is:

log₁₀P = 7.4550-1880.l/T (T = abs. temp.),

from which the latent heat of evaporation is calculated to be 63.9 cals./gm. Or 8626.5 cals./mol., and the molecular boiling-point elevation constant is 52.9. Under the foregoing conditions sulphur monochloride is a stable, well-defined compound.

The monochloride exhibits a slight tendency to molecular dissociation, but the effect is so small that both in the dissolved and in the gaseous condition the molecular weight agrees with the formula S₂Cl₂. As it does not combine with bromine, ebullioscopic determinations of the molecular weight have been made in this solvent, and also even in liquid chlorine.

The compound is exothermic:

S₂(rhombic) + Cl₂(gas) = S₂Cl₂(liquid) + 14,260 calories.

Sulphur monochloride is soluble in many organic solvents, e.g. carbon disulphide, benzene, carbon tetrachloride and light petroleum; it also possesses solvent properties, dissolving sulphur, very readily on warming and slowly depositing much of the solid on cooling; the molecular heat of solution of sulphur in sulphur monochloride is from -1300 to -3000 calories. Chlorine, bromine and iodine are also dissolved, the two latter without chemical change; with chlorine, however, combination occurs readily at the ordinary temperature, although at the boiling-point of liquid chlorine simple solution occurs primarily. The heat of solution of chlorine in sulphur monochloride is shown in the following equations:

S₂Cl₂(liq.) + Cl₂(gas) = 2SCl₂(dissolved in S₂Cl₂) + 9800±400 calories,
and
S₂Cl₂(liq.) + 3Cl₂(gas) = 2SCl₄(dissolved in mixture of S₂Cl₂ and SCl₂) + 12,000 to 14,000 calories.

Sulphur monochloride will not burn under ordinary conditions in the air, but when passed through a red-hot tube in the vapour condition and mixed with air or oxygen, combustion occurs, accompanied by a greenish-blue flame and the formation of sulphur dioxide and trioxide, together with chlorine. In contact with water the monochloride undergoes decomposition according to the equation:

S₂Cl₂ + 2H₂O = H₂S + 2HCl + SO₂.

Even in the presence of a large excess of water decomposition only proceeds to the extent of 93.68 per cent., being partly restrained by the presence of the hydrogen chloride formed. The other factor which influences the extent of the action is the sulphur formed by subsequent reaction between the hydrogen sulphide and the sulphur dioxide. Polythionic acids are formed in solution and the sulphur which separates encloses undecomposed chloride and may also dissolve in it.

With phosphorus the reaction products vary according to the conditions. In the presence of an excess of monochloride, phosphorus sulphochloride and free sulphur are obtained, whereas if the monochloride is introduced into an excess of molten phosphorus, the products are the trichloride and sulphide of phosphorus, some red phosphorus also being formed.

With regard to the general reaction of sulphur monochloride with metals, Domanicki has stated that univalent metals do not react, and bivalent metals, with the exception of mercury, if they react at all, do so with much greater difficulty than ter- or quadri-valent metals. There are other important exceptions to this rule, however; silver is slightly attacked by the monochloride, whilst copper and manganese are very considerably attacked. On the other hand, cobalt and chromium are unaffected, and it has been suggested that drums made of alloy or plated steel provide suitable containers for sulphur monochloride. The corrosive action is greatly accelerated by the presence of dry ether, with which the metallic chlorides form complexes, thus enhancing the thermal effect of the action.

Many metallic oxides, especially those yielding volatile chlorides, are attacked by the vapour of sulphur monochloride, and the reaction forms a useful method for the conversion of such oxides into the corresponding anhydrous chlorides. Assuming a bivalent metal M, the change is:

2MO + 2S₂Cl₂ = 2MCl₂ + SO₂ + 3S.

Such treatment can be extended to mineral substances, some of which at 800° C. under the action of sulphur monochloride vapour are easily convertible into the corresponding metallic chlorides.

Sulphur trioxide and sulphur monochloride react to give pyro-sulphuryl chloride, S₂O₅Cl₂, whilst sulphates when heated in the monochloride vapour are converted into chlorides in the following manner:

Na₂SO₄ + 2S₂Cl₂ = SO₂Cl₂ + 2NaCl + SO₂ + 3S.

Under the influence of the silent discharge sulphur monochloride vapour is reduced by hydrogen to hydrogen chloride and sulphur or hydrogen sulphide. Hydriodic acid effects a similar reduction at the ordinary temperature, the products being hydrochloric acid, sulphur and hydrogen sulphide, together with iodine.

Phosphorus trichloride reacts with sulphur monochloride, iodine acting as a catalyst; the products are phosphorus pentachloride and phosphorus sulphochloride:

3PCl₃ + S₂Cl₂ = PCl₅ + 2PSCl₃.

Antimony pentachloride reacts to form the compound SbCl₅.SCl₄, which may be obtained as amber-coloured crystals.

When an ice-cold solution of ammonia in chloroform is added to a solution of sulphur monochloride in the same solvent, sulphides of nitrogen are formed, the primary reaction being:

6S₂Cl₂ + 16NH₃ = N₄S₄ + 12NH₄Cl + 8S.

The tetrasulphide may be precipitated by the addition of alcohol, and by concentration of the mother-liquor the pentasulphide, N₂S₅, and the hexasulphamide, S₆NH₂, may be obtained.

Sulphur monochloride reacts vigorously with many organic compounds, generally exerting a chlorinating or a sulphurating action. Thus it converts ethyl alcohol into ethyl chloride, aniline into dithio-phenylamine, and anthracene into 9-anthryldithioehloride, C₁₄H₉S₂Cl. Even ether is slowly decomposed by sulphur monochloride. The sodium salts of the fatty acids are converted into their corresponding acid chlorides.

The purity of a sample of sulphur monochloride may be determined by heating with aqueous sodium hydroxide (2N) for four hours in a long-necked flask on a water-bath and, after cooling, adding a few c.c. of 30 per cent, hydrogen peroxide. The mixture is then reheated for not more than half an hour, slightly acidified with nitric acid, and diluted to a known volume. The chlorine may then be estimated volumetrically and the sulphur gravimetrically in aliquot portions.

Sulphur monochloride is extensively used in the vulcanisation of caoutchouc at the ordinary temperature by a process discovered by Parkes; the chemical reaction appears to involve merely the addition of sulphur chloride to an unsaturated hydrocarbon. A similar reaction is involved in the manufacture of "mustard gas," dichlorodiethyl- sulphide, from ethylene and sulphur monochloride:

2C₂H₄ + S₂Cl₂ = S(C₂H₄Cl)₂ + S.

The monochloride is also employed in the manufacture of carbon tetrachloride and certain anhydrous inorganic chlorides. Its use has also been suggested in the refining of sugar.

Although the structure S:SCl₂ is possible for sulphur monochloride, there appears to be little real evidence in confirmation of such an analogy with thionyl chloride, O:SCl₂. The relation of sulphur monochloride to the other chlorides of sulphur, and the fact that in its interaction with organo-magnesium compounds products are obtainable which are known definitely to have the constitution R•S•S•R, where R represents an organic radical, are entirely in support of a structure Cl•S•S•Cl, analogous to the probable structure of hydrogen disulphide.