Thionyl Chloride, SOCl₂

Thionyl Chloride, SOCl₂, in an impure condition was first obtained by Persoz and Bloch in 1849 by the interaction of sulphur dioxide and phosphorus pentachloride. Further early investigations were made by Schiff in 1857 and by Carius between 1856 and 1864.

1. The reaction by which the substance was discovered can be effected conveniently by passing sulphur dioxide over phosphorus pentachloride, when the resulting thionyl chloride can subsequently be separated from the accompanying phosphoryl chloride by fractional distillation:
   
   SO₂ + PCl₅ = SOCl₂ + POCl₃.
2. Thionyl chloride can be obtained by the interaction of sulphur chlorides with sulphur trioxide:
   
   SCl₂ + SO₃ = SOCl₂ + SO₂,
   SCl₄ + SO₃ = SOCl₂ + SO₂ + Cl₂.
   
   It is convenient, however, to use the commoner monochloride, with which a similar reaction readily occurs at 75° to 80° C.; the sulphur produced can be reconverted continuously into monochloride by means of a current of chlorine, and the formation of the thionyl chloride can be assisted by the addition of suitable catalysts, such as antimony trichloride or mercuric chloride.
3. Another process which may be regarded in some ways as a modification of (2) is based on the interaction of sulphur or sulphur monochloride with chlorosulphonic acid in a stream of chlorine, the same catalysts as before again being applicable:
   
   S₂Cl₂ + Cl₂ + 2Cl.SO₂.0H-2S0Cl₂ + 2HCl + 2SO₂.
4. Other chemical changes yielding thionyl chloride include the action of phosphorus pentachloride on sulphuryl chloride,4 sulphites and thiosulphates; these processes may be regarded as modifications of (1):
   
   K₂SO₃ + 2PCl₅ = 2KCl + SOCl₂ + 2POCl₃.

Thionyl chloride is also formed by the oxidation with chlorine monoxide of sulphur in carbon disulphide or even of carbon disulphide itself, and in the interaction of carbonyl chloride with sulphur dioxide at temperatures above 200° C.,

COCl₂ + SO₂ = CO₂ + SOCl₂,

which reaction occurs readily if the gaseous mixture is passed over a suitable " contact" material, e.g. heated wood charcoal.

The spontaneous decomposition of sulphur oxytetrachloride, S₂O₃Cl₂, also gives rise to thionyl chloride.

Thionyl chloride is a colourless, refractive liquid, of density 1.676 at 0° C., with freezing-point -104.5° C. and boiling-point 79° C., under atmospheric pressure. The vapour badly attacks the mucous membranes and possesses an odour recalling that of sulphur dioxide. Up to 150° C. the vapour density is normal, but above this temperature decomposition sets in, finally causing a value only two-thirds of the normal (see the following).

With respect to its elements, the substance is exothermic to the extent of approximately 47200 calories per gram-molecule; the latent heat of vaporisation is 54.45 calories per gram and the specific heat at ordinary temperatures is 0.2425. When dissolved in benzene or chloroform, thionyl chloride possesses practically a normal molecular weight, but when used as a solvent it permits ionisation. Its dielectric constant is 9.05 at 22° C.

As mentioned before, thionyl chloride is decomposed by heat, and at a dull redness it gives chlorine, sulphur dioxide and sulphur monochloride:

4SOCl₂ = 3Cl₂ + 2SO₂ + S₂Cl₂.

Water in the cold causes hydrolysis to sulphurous acid and hydrogen chloride, but with hot water some sulphur and sulphuric acid may also be formed:

SOCl₂ + 2H₂O = H₂SO₃ + 2HCl.

The silent electric discharge in the presence of hydrogen causes transformation of thionyl chloride into sulphur monochloride and sulphur dioxide, with hydrogen chloride:

4SOCl₂ + 3H₂ = S₂Cl₂ + 2SO₂ + 6HCl.

At 180° C. thionyl chloride converts sulphur into sulphur monochloride, with simultaneous formation of sulphur dioxide, this change in all probability being dependent on the afore-mentioned thermal decomposition:

2SOCl₂ + 3S = 2S₂Cl₂ + SO₂.

Under similar conditions thionyl chloride reacts with both red and yellow phosphorus with formation of phosphorus trichloride according to the equation:

4SOCl₂ + 2P = 2PCl₃ + 2SO₂ + S₂Cl₂.

Prolonged heating tends to produce the pentachloride:

3PCl₃ + 4SOCl₂ = 3PCl₅ + 2SO₂ + S₂Cl₂.

At higher temperatures phosphorus trichloride reacts with thionyl chloride according to the equation:

SOCl₂ + 3PCl₃ = PCl₅ + PSCl₃ + POCl₃.

Selenium and tellurium are converted into their respective tetrachlorides by thionyl chloride, whilst gold, mercury, bismuth, arsenic, antimony, tin and iron give a mixture of the metallic chloride with sulphur dioxide and sulphur monochloride, for example:

2Bi + 4SOCl₂ = 2BiCl₃ + S₂Cl₂ + 2SO₂.

Where the metal can exist in two states of valency, as in the case of the three last-named metals, the products depend to some extent on the relative quantities of the reagents; with excess of metal the course of the reaction is as follows, antimony being taken as a typical case:

6Sb + 6SOCl₂ = 4SbCl₃ + Sb₂S₃ + 3SO₂,

whilst with an excess of thionyl chloride this reaction is followed by another:

3SbCl₃ + 4SOCl₂ = 3SbCl₅ + S₂Cl₂ + 2SO₂.

Towards metallic oxides the behaviour of thionyl chloride is similar to that of sulphur monochloride, which is perhaps hardly surprising in view of the course of its thermal decomposition. The reaction is fairly general, the oxide being converted into the corresponding anhydrous chloride. From the action of the chloride on zinc oxide (at 150° C.), cadmium oxide (at 200° C.), arsenious oxide (up to 200° C.), antimony trioxide (at room temperature), bismuth trioxide (at 150° to 200° C.), ferric oxide (at 150° C.), magnesium oxide (at 150° to 200° C.), cupric oxide (at 200° C.) and cuprous oxide (at 200° C.), it may be concluded that the main reaction, assuming a bivalent metal, M, is as follows:

MO + SOCl₂ = MCl₂ + SO₂.

If the metal is capable of forming a higher chloride, a further reaction can occur, e.g.:

3MCl₂ + 4SOCl₂ = 3MCl₂ + 2SO₂ + S₂Cl₂.

Calcium, strontium, aluminium and stannic oxides are not attacked.

Peroxides react vigorously with thionyl chloride, forming sulphuryl chloride. The metal remains either as chloride or as a mixture of chloride and sulphate, according to the proportion of thionyl chloride. With thionyl chloride in excess the reaction takes place according to the equation (using barium peroxide):

BaO₂ + 2SOCl₂ = BaCl₂ + SO₂ + SO₂Cl₂,

whilst with excess of barium peroxide the reaction is:

2BaO₂ + 2SOCl₂ = BaCl₂ + BaSO₄ + SO₂Cl₂.

Thionyl chloride has little action on selenium at room temperature, but on heating gives the tetrachloride:

2SOCl₂ + Se = SeCl₂ + SO₂ + S.

With selenium dioxide the reaction is:

2SOCl₂ + SeO₂ = SeCl₂ + 2SO₂.

With tellurium or tellurium dioxide, thionyl chloride gives tellurium tetrachloride if SOCl₂ is in excess, or the dichloride if the element or its dioxide is in excess. With mercury at 150° C. in a sealed tube the reaction may either be

Hg + 4SOCl₂ = HgCl₂ + 2SO₂Cl₂ + S₂Cl₂,
or
3Hg + 4SOCl₂ = 3HgCl₂ + 2SO₂ + S₂Cl₂,

according to the proportions used. With mercuric oxide in a sealed tube at 160° C. and thionyl chloride in excess, the reaction is:

HgO + 5SOCl₂ = HgCl₂ + 3SO₂Cl₂ + S₂Cl₂.

If the SOCl₂ is not in excess, the reaction is probably:

HgO + SOCl₂ = HgCl₂ + SO₂.

Hydrogen sulphide is gradually oxidised by thionyl chloride, the process being greatly accelerated by the presence of aluminium chloride; the products are sulphur dioxide, sulphur and hydrogen chloride, together with sulphur monochloride if the thionyl chloride is in excess:

2SOCl₂ + 2H₂S = 4HCl + SO₂ + 3S,
2SOCl₂ + H₂S = S₂Cl₂ + SO₂ + 2HCl.

When heated in a sealed tube with thionyl chloride at 150° to 200° C., many metallic sulphides in a similar manner yield the metallic chloride, together with sulphur monochloride and sulphur dioxide:

MS + 2SOCl₂ = MCl₂ + SO₂ + S₂Cl₂.

Most mineral sulphides may be decomposed by this means, a few hours' heating being sufficient in the case of easily decomposed sulphides such as pyrites, cinnabar, galena, orpiment, mispickel and stibnite. Pyrargyrite, proustite, covellite, sphalerite and tetrahedrite require from one to two days, whilst argentite, molybdenite and cobaltite are not attacked by thionyl chloride under these conditions.

Pure sulphuric acid gradually yields chlorosulphonic acid and pyro-sulphuryl chloride by concurrent independent reactions on treatment with thionyl chloride. Nitric acid vigorously oxidises thionyl chloride to sulphuric acid, probably with intermediate formation of nitryl chloride,

NO₂Cl. In the absence of moisture silver nitrate yields nitrosulphonyl chloride:

SOCl₂ + AgNO₃ = NO₂.SO₂.Cl + AgCl.

As suggested by its behaviour towards sulphuric and nitric acids, thionyl chloride is of value as a reagent for the replacement of hydroxyl groups by chlorine, and amongst the organic compounds it finds frequent application instead of the commoner chlorides or oxychloride of phosphorus.

Even at the ordinary temperature hydrogen iodide is readily decomposed by thionyl chloride, giving hydrogen chloride, iodine, sulphur and sulphur dioxide, but hydrogen bromide undergoes a double decomposition with formation of thionyl bromide and hydrogen chloride:

SOCl₂ + 2HBr = SOBr₂ + 2HCl.

Thionyl chloride reacts with ammonia giving rise to various complex products, the nature of which varies with the conditions. With amino-sulphonic acid at 100° C. ammoniumTchlorosulphonate is formed.

The constitution of thionyl chloride, judged by the various methods by which the substance can be prepared, can hardly be other than , and this view is confirmed by the formation of sulphoxides when the substance interacts with organo-magnesium compounds, magnesium phenyl bromide, for example, yielding diphenyl sulphoxide, (C₆H₅)₂S:O. On account of the relationship of thionyl chloride to sulphurous acid, the molecular structure of the chloride possesses peculiar interest.