Sulphuryl Chloride, SO₂Cl₂

As would be expected, it is possible to obtain an Sulphuryl Chloride, SO₂Cl₂, corresponding with sulphuric acid, SO₂(OH)₂, but the chloride cannot be prepared by the action of phosphorus pentachloride on sulphuric acid or sulphur trioxide, these reagents yielding pyrosulphuryl chloride.

Chlorine and sulphur dioxide will only unite under some accelerating influence. It was by the interaction of these gases in sunlight that sulphuryl chloride was first obtained by Regnault in 1838.

Bone charcoal or activated wood charcoal is a convenient accelerator, combination occurring instantly. If the vessel in which the reaction is carried out is cooled to 30° C., the sulphuryl chloride is condensed and may be drained away as rapidly as it is formed. The terpene hydrocarbons, especially pinene and limonene, also certain ethers, phenols and esters, are effective catalysts, as also is camphor.

The formation of sulphuryl chloride is favoured by low temperature and its decomposition by high temperature:

SO₂ + Cl₂ ⇔ SO₂Cl₂.

Both these reactions are .catalysed by the foregoing catalysts. For the efficient preparation of the chloride, the dry reacting gases are brought together at ordinary temperatures in the presence of a catalyst such as "Norit," a highly activated powdered carbon. The mixture is then cooled to -10° C. to ensure complete combination, filtered from the catalyst and carefully distilled, the heat being applied to the liquid for as short a time as possible.

At 120° C. in a sealed tube boron trichloride and sulphur trioxide react with formation of sulphuryl chloride:

2BCl₃ + 4SO₃ = 3SO₂Cl₂ + B₂O₃.SO₃.

A convenient method for the laboratory preparation of sulphuryl chloride consists in boiling chlorosulphonic acid with about one per cent, of mercury or mercuric sulphate under a reflux condenser kept at a temperature of 70° C. in order to return to the flask any unchanged chlorosulphonic acid:

2Cl.SO₂.OH = SO₂Cl₂ + H₂SO₄.

Any unchanged chlorosulphonic acid in the distillate can be removed by making use of its much more rapid hydrolysis with ice-cold water, the residual liquid being dried and redistilled.

Sulphuryl chloride is a colourless, fuming liquid, with an extremely pungent odour. D²⁰₄ = 1.6674; n²⁰_D = 1.4437. It boils at 69.1° C. at 760 mm. pressure, and freezes at -46° C. The vapour density is normal at first, but when the chloride is kept, even at 100° C., its vapour commences to dissociate into sulphur dioxide and chlorine. At 200° C. dissociation is almost complete. When dissolved in benzene the substance shows a molecular weight corresponding with SO₂Cl₂. At ordinary temperatures the specific heat is 0.233, the latent heat of evaporation 52.4 calories per gram, and the heat of formation from the elements approximately 89,540 calories per gram-molecule. The dielectric constant at 20° C. is 8.5. As a solvent, the ebullioscopic constant of sulphuryl chloride has been found to have

the value 45; salts dissolved in the chloride are found to undergo ionic dissociation.

At a dull red heat sulphuryl chloride vapour is completely decomposed into sulphur dioxide and chlorine. At 320° C. the reaction appears to be of the first order, proceeding entirely in the gas phase; at lower temperatures, however, reaction takes place on the wall of the containing vessel.

Water causes sulphuryl chloride to decompose into sulphuric and hydrochloric acids, but with only very little water, or better, with sulphuric acid, chlorosulphonic acid is obtained:

SO₂Cl₂ + 2H₂O = H₂SO₄ + 2HCl.
SO₂Cl₂ + H₂SO₄ = 2Cl.SO₂.OH.

At 200° C. sulphuryl chloride converts sulphur into monochloride, this result probably being preceded by dissociation of the sulphuryl chloride into chlorine and sulphur dioxide. In the presence of aluminium chloride this reaction can be effected even at the ordinary temperature, and aluminium chloride is known.to favour the dissociation of sulphuryl chloride:

SO₂Cl₂ + 2S = S₂Cl₂ + SO₂.

In the presence of aluminium chloride, iodine also reacts easily with sulphuryl chloride:

SO₂Cl₂ + 2I = SO₂ + 2ICl,
2SO₂Cl₂ + 2ICl = 2SO₂ + 2ICl₃.

Hydrogen sulphide is attacked according to the equations:

H₂S + SO₂Cl₂ = 2HCl + SO₂ + S,
2H₂S + SO₂Cl₂ = 2H₂O + S₂Cl₂ + S;

whilst hydrogen bromide and hydrogen iodide are acted upon vigorously with formation of sulphur dioxide and the free halogen:

SO₂Cl₂ + 2HBr = SO₂ + Br₂ + 2HCl.

Phosphorus (red more readily than yellow), arsenic, antimony, mercury, iron, gold and platinum are converted into chlorides, with liberation of sulphur dioxide, e.g.

Hg + SO₂Cl₂ = HgCl₂ + SO₂;

with the mercury in excess:

2Hg + SO₂Cl₂ = 2HgCl + SO₂.

In ether solution sulphuryl chloride reacts with zinc, giving zinc chloride and zinc sulphoxylate, ZnSO₂.

Phosphorus pentachloride effects a gradual removal of one atom of oxygen from sulphuryl chloride, with formation of thionyl chloride,

PCl₅ + SO₂Cl₂ = POCl₃ + SOCl₂ + Cl₂,

whilst phosphorus trichloride yields the same products, excepting chlorine.

Lead dioxide is converted vigorously into lead chloride and sulphate by the vapour of sulphuryl chloride, oxygen being liberated, whilst mercuric oxide (red at 160° to 180° C., yellow at 150° C.) with excess of sulphuryl chloride gives mercuric chloride and sulphur trioxide:

HgO + SO₂Cl₂ = HgCl₂ + SO₃.

With excess of mercuric oxide, mercuric sulphate is also formed.

Selenium is attacked rapidly by sulphuryl chloride:

Se + 2SO₂Cl₂ = SeCl₂ + 2SO₂.

The chloride has no action on selenium dioxide even at high temperatures or under great pressure. Sulphuryl chloride reacts with tellurium; with the sulphuryl chloride in excess the reaction is:

Te + 2SO₂Cl₂ = TeCl₂ + 2SO₂,

whilst with the tellurium in excess, TeCl₂ is obtained. Tellurium dioxide is not acted upon in the cold, but when heated in a sealed tube with the chloride a variety of crystalline products may be formed: 3TeO₂.4SO₂Cl₂, 5TeO₂.9SO₂Cl₂, TeO₂.2SO₂Cl₂, and 2TeO₂.5SO₂Cl₂.

The reaction between sulphuryl chloride and ammonia is complex, various products being obtained under different conditions; the products include iminosulphamide, NH₂.SO₂.NH.SO₂.NH₂, which behaves as a monobasic acid, trisulphimide and sulphomelide, both acidic substances of composition (SO₂.NH)₃, and sulphamide, SO₂(NH₂)₂, which is also acidic.

Like thionyl chloride, sulphuryl chloride is able to replace the hydroxyl groups of organic substances by chlorine.

In view of the possibility of sexavalent sulphur, as demonstrated by the existence of sulphur hexafluoride, there is very little difficulty in accepting the formula for sulphuryl chloride.