Nitrosulphonic Anhydride, S₂O₅(NO₂)₂

Nitrosulphonic anhydride, or nitrous-pyrosulphuric anhydride (the choice of name depending on the view entertained as to the structure of the NO₂-group), is formed during the decomposition of nitrosulphonic acid by heat, but it may more conveniently be obtained by the action of dry nitric oxide on sulphur trioxide:

2NO + 3SO₃ = NO₂.SO₂.O.SO₂.NO₂ + SO₂.

It is also formed by the action of nitrogen peroxide on liquid sulphur dioxide under pressure:

2N₂O₄ + 2SO₂ = S₂O₅(NO₂)₂ + N₂O₃.

With both react ants in the liquid state the reaction proceeds according to the equation:

1.5N₂O₄ + 2SO₂ = S₂O₅(NO₂)₂ + NO.

The pressure increases slowly at first but after a time a considerable increase suddenly occurs. The anhydride is also formed when the two gases react, only very slowly at ordinary temperatures, but more rapidly at higher temperatures. The reaction is never complete but goes somewhat further in the presence of platinised asbestos. The anhydride may also be prepared by the action of liquid nitrogen tetroxide on pyrosulphuric acid or pyrosulphuryl chloride.

Nitrosulphonic anhydride is a white, crystalline solid, of density 2.14. On heating it undergoes partial decomposition; it begins to melt at 217° C. to a yellow liquid and distils at about 360° C. The heat of formation, calculated from the heat of dissolution in potassium hydroxide, is 112 Calories:

2SO₂ liq. + l.5N₂O₄ liq. = S₂N₂O₉ solid + NO .gas + 112 Calories.

As would be expected from a substance of this nature, water causes rapid decomposition into sulphuric acid and the decomposition products of nitrous acid, whilst sulphuric acid dissolves the substance with formation of nitrosulphonic acid, which crystallises on cooling. With sulphur trioxide the anhydride yields a stable complex of composition 2(S₂N₂O₉).SO₃. Gaseous ammonia causes the anhydride to melt and decompose slowly with formation of nitrogen and ammonium hydrogen sulphate:

(NO₂)₂S₂O₅ + 4NH₃ = 2NH₄HSO₄ + H₂O + 2N₂.

The modes of preparation, particularly from pyrosulphuric acid and pyrosulphuryl chloride, and the general properties of nitrosulphonic anhydride, suggest the structure O.NO.SO₂.O.SO₂.O.NO.