Carbonyl Sulphide, COS

Carbonyl Sulphide or Carbon Oxysulphide, COS, discovered by Than, is formed synthetically when a mixture of sulphur vapour and carbon monoxide is passed through a hot tube. The action is reversible,

CO + S ⇔ COS,

so that the proportion of carbonyl sulphide formed depends upon the temperature and rate of passage of the gaseous mixture. Attempts to synthesise carbonyl sulphide by means of the electric spark seem to give negative results.

Carbonyl sulphide is also formed by the interaction of carbon disulphide and sulphur trioxide, thus:

CS₂ + 3SO₃ = COS + 4SO₂.

When sulphur vapour and air are passed over a red-hot mixture of clay and carbon, and also when sulphur dioxide is passed over red-hot carbon, carbonyl sulphide is formed:

4SO₂ + 9C = 6CO + 2COS + CS₂.

Some thiocarbonates yield carbonyl sulphide on decomposition, thus:

+ HCl = COS + C₂H₅OH + KCl.

At 270° C. carbonyl chloride and cadmium sulphide interact according to the equation:

COCl₂ + CdS = COS + CdCl₂.

An important method of preparing carbonyl sulphide consists in the decomposition of a thiocyanate with moderately concentrated (14N) sulphuric acid, the liberated thiocyanic acid being hydrolysed thus:

HCNS + H₂O = COS + NH₃.

The carbonyl sulphide is evolved at 20° C. together with hydrocyanic acid, formic acid and carbon disulphide. The gas is purified by passing through concentrated aqueous caustic potash to absorb the acid vapours, and the carbon disulphide is absorbed in a mixture of triethylphosphine, pyridine and nitrobenzene. After drying with sulphuric acid, the gas may be further purified by liquefaction or absorption in toluene.

Carbonyl sulphide may also be obtained by the action of hydrochloric acid on commercial ammonium thiocarbamate, according to the equation:

NH₄.CO.S.NH₂ + 2HCl = COS + 2NH₄Cl.

In this case the gas may be purified by bubbling through 33 per cent, caustic soda solution in order to absorb carbon dioxide and hydrogen sulphide and dried with calcium chloride and phosphorus pentoxide. It is then condensed by means of liquid air, and finally fractionated.

Pure carbonyl sulphide is a colourless, odourless gas, which is slowly decomposed by water. When dry it is stable, even in sunlight, and if kept over sulphuric acid it undergoes only slight decomposition. Its density at -87° c. is 1.24. It melts at -138.2° C. and boils at -50.2° C. under a pressure of 760 mm.

1 volume of water dissolves 0.54 volumes of COS at 20° C.
1 volume of alcohol dissolves 8.0 volumes of COS at 22° C.
1 volume of toluene dissolves 15.0 volumes of COS at 22° C.

The thermal decomposition of carbonyl sulphide has been investigated. The products of dissociation may be carbon monoxide and sulphur on the one hand, or carbon dioxide and carbon disulphide on the other:

1. COS ⇔ CO + S,
2. 2COS ⇔ CO₂ + CS₂.

At 800° C. reaction (2) appears to proceed slowly in either direction, while (1) is very rapid. The fact that the degree of dissociation in (1) is independent of the amounts of carbon dioxide and carbon disulphide present, shows that carbon monoxide and sulphur are primary products of the decomposition of carbonyl sulphide and are not formed secondarily from the carbon dioxide and carbon disulphide. At temperatures below 400° C. decomposition according to equation (1) is not evident, while at 900° C. it reaches a maximum (64 per cent.); reaction (2) reaches a maximum at about 600° C., at which point 43 per cent, of the carbonyl sulphide is decomposed in this way and 16 per cent, according to reaction (1). The carbon monoxide equilibrium depends upon the pressure, whilst the carbon dioxide equilibrium does not. Nearly all the reactions involved in the thermal decomposition of carbonyl sulphide depend greatly on catalytic influences. Quartz is a pronounced catalyst for reaction (2), but has little influence on reaction (1). Carbonyl sulphide is comparatively rapidly decomposed in quartz vessels, but is stable when kept in glass apparatus. The viscosity of gaseous carbonyl sulphide is as follows:

η₁₅ = 1.200×10^-4 C.G.S. units,
and
η₁₀₀ = 1.554×10^-4 C.G.S. units;

by extrapolation from Sutherland's formula, η₀ = 1.135×10^-4 C.G.S. units. The mean area (Å) which the molecule presents in mutual collision is 1.06×10^-15 cm².

The physiological effects of carbonyl sulphide are very similar to those of nitrous oxide.

When the pure gas is passed through a saturated solution of barium hydroxide or copper sulphate, no opalescence or precipitate is produced for at least half a minute; if any carbon dioxide is present, however, the solution becomes milky at once. According to Weeldenburg there is no reaction between carbonyl sulphide and copper sulphate in neutral or acid solution, nor is there any reaction with iodine or ethereal tri-ethylphosphine.

Carbonyl sulphide burns with a blue flame, forming carbon dioxide and sulphur dioxide. With air it forms a mixture which is slightly explosive except when quite dry. The explosive limits lie between 11.9 and 28.5 per cent, of carbonyl sulphide. A white-hot platinum wire completely decomposes the sulphide into carbon monoxide and sulphur.

Water slowly decomposes carbonyl sulphide, forming carbon dioxide and hydrogen sulphide:

COS + H₂O = CO₂ + H₂S.

According to Buchbock the reaction proceeds in two stages, thiol-carbonic acid being an intermediate product:

COS + H₂O ⇔ ⇔ CO₂ + H₂S.

Aqueous solutions of the caustic alkalis act only slowly with the oxysulphide to form thiolcarbonates, which, however, soon decompose into carbonate and hydrosulphide:



With alcoholic potash the reaction is more rapid.

Heated mercury, copper, iron and silver remove sulphur from carbonyl sulphide; cuprous chloride reacts according to the equation:

COS + Cu₂Cl₂ + H₂O = Cu₂S + 2HCl + CO₂;

chlorine forms phosgene, COCl₂, together with "sulphur dichloride":

COS + 2Cl₂ = COCl₂ + SCl₂.