Chlorosulphonic Acid, Cl.SO₂,OH

In 1854 the discovery of chlorosulphonic acid by Williamson, by the interaction of hydrogen chloride and sulphur trioxide, was of especial interest. Ideas of valency were immature and undeveloped, and for the systematisation of the knowledge of organic and inorganic compounds reference was made to various types such as HCl, H₂O and H₃N, from which other substances could be regarded as derived by substitution of one or more of the hydrogen atoms by other atoms or groups of atoms. Chlorosulphonic acid was the first example of a compound derived simultaneously from two types, namely from hydrogen chloride and water by the replacement of a hydrogen atom from each by the group SO₂.

The following methods are available for the preparation of chlorosulphonic acid: (1) The substance can be obtained by the gradual action of moisture (e.g. atmospheric moisture) on sulphuryl chloride; a similar reaction occurs with sulphuric acid and is more easily regulated:

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

To the former reaction is also to be ascribed the formation of chlorosulphonic acid when moist chlorine and sulphur dioxide are passed over platinum at a red heat.

(2) Concentrated or fuming sulphuric acid also gives rise to chlorosulphonic acid when treated with phosphorus pentachloride:

H₂SO₄ + PCl₅ = Cl.SO₂.OH + POCl₃ + HCl;

other acid chlorides such as phosphorus oxychloride, phosphorus trichloride, sulphuryl chloride, pyrosulphurvl chloride, sulphur oxytetrachloride, and even sulphur monochloride, give a similar result.

(3) Even chlorine alone will act on sulphuric acid, giving chlorosulphonic acid, as also will hydrogen chloride on sulphur trioxide or on a mixture of sulphuric acid and phosphorus pentoxide. Indeed, the interaction of hydrogen chloride and sulphur trioxide in the presence of sulphuric acid provides the most convenient method for preparing chlorosulphonic acid, the product being purified by distillation in an atmosphere of hydrogen chloride, followed by fractional crystallisation:

Cl₂ + H₂SO₄ = Cl.SO₂.OH + HCl;
HCl + SO₃ = Cl.SO₂.OH.

Chlorosulphonic acid is a colourless fuming liquid of sharp, unpleasant odour. It boils at 151° to 152.5° C. at 765 mm., or at 74° to 75° C. at 19 mm., and it melts at -81°to -80°C.; D⁰₄ = 1.784, D²⁰₄ = 1.753. At the boiling-point the vapour density is normal, but marked dissociation occurs at higher temperatures, e.g. at 200° C., with formation of sulphur trioxide and hydrogen chloride. Referred to solid sulphur trioxide and gaseous hydrogen chloride, the formation of chlorosulphonic acid is exothermic to the extent of 14.4 Calories per gram- molecule; the specific heat at ordinary temperatures is 0.282 and the latent heat of vaporisation 12.8 Calories per gram-molecule.

When heated strongly, the vapour of chlorosulphonic acid undergoes decomposition into sulphur trioxide and hydrogen chloride or, at higher temperatures, into sulphur dioxide, sulphuric acid and chlorine; under suitable conditions, e.g. at 180° to 210° C. in a sealed tube or especially readily in the presence of certain catalysts such as mercury or mercuric chloride, platinum chloride, copper, iodine or sulphur, the liquid substance undergoes disruption into sulphuryl chloride and sulphuric acid.

Water causes very vigorous hydrolysis to sulphuric acid and hydrogen chloride, the action being much more rapid than with pyrosulphuryl chloride:

Cl.SO₂.OH + H₂O = HCl + SO₂(OH)₂.

Hydrogen sulphide is readily decomposed by chlorosulphonic acid with formation of sulphur, sulphur monochloride and sulphuric acid; the reaction may be expressed as

2H₂S + 2Cl.SO₂.OH = S₂Cl₂ + 2H₂O + S + H₂SO₄,

but is probably more complex.

Chlorosulphonic acid possesses a strong chlorinating power and converts sulphur, phosphorus, arsenic, antimony and tin into the corresponding chlorides, sulphur dioxide together with sulphuric and hydrochloric acids being simultaneously produced. With sulphur and with yellow phosphorus interaction occurs at the ordinary temperature, often becoming uncontrollable with the latter element.

With sulphuric acid and phosphorus pentachloride, respectively, fuming sulphuric acid and pyrosulphuryl chloride are obtained; in the latter case the effect is one of dehydration, and a similar result is producible with phosphorus pentoxide:

Cl.SO₂.OH + H₂SO₄ = O(SO₂.OH)₂ + HCl;
2Cl.SO₂.OH + PCl₅ = O(SO₂Cl)₂ + POCl₃ + 2HCl;
2Cl.SO₂.OH + P₂O₅ = O(SO₂Cl)₂ + 2HPO₃.

Potassium sulphate, when warmed with chlorosulphonic acid, yields potassium pyrosulphate, the reaction being analogous to that of sulphuric acid with the chloro-acid; silver nitrate is vigorously converted into silver chloride, with concurrent formation of nitrosulphonic acid.

At the ordinary temperature chlorosulphonic acid dissolves sodium chloride, displacing hydrogen chloride and forming sodium chlorosulphonate, this reaction being due to its acidic nature. It is of interest that ammonium chlorosulphonate can be synthesised by the action of sulphuryl chloride on aminosulphonic acid at 100° C.

The simple acidic nature of chlorosulphonic acid is also manifested in the existence of corresponding ethereal salts, the ethyl ester Cl.SO₂.OC₂H₅ being obtainable indirectly by the action of phosphorus penta-chloride on ethyl hydrogen sulphate (ethylsulphuric acid), C₂H₅O.SO₂.OH, or by the interaction of sulphuryl chloride and ethyl alcohol:

C₂H₅O.SO₂.OH + PCl₅ = Cl.SO₂.OC₂H₅ + POCl₃ + HCl;
SO₂Cl₂ + C₂H₅OH = Cl.SO₂.OC₂H₅ + HCl.

From its general behaviour it is clear that chlorosulphonic acid is closely related to sulphuric acid. Like the latter acid, but with more vigour, it attacks aromatic hydrocarbons with formation of organic sulphonic acids of the structure R.SO₂.OH and sulphones of the structure , where R represents an aromatic organic radical such as phenyl (C₆H₅). From these and other reactions the conclusion may be drawn that in chlorosulphonic acid the chlorine atom must be directly attached to sulphur, and therefore that the constitution must be represented as