Hydrogen Disulphide, H₂S₂

Hydrogen Disulphide, H₂S₂, in addition to being obtained from the distillation of crude "hydrogen persulphide," is also formed when hydrogen trisulphide is distilled at 100° C. under a pressure of 20 mm.; approximately one-third of the trisulphide is converted into disulphide, whilst the remainder passes into hydrogen sulphide and sulphur. The disulphide, therefore, is the most stable of the hydrogen polysulphides towards heat, and can actually be distilled under atmospheric pressure at 71° to 75° C. with only partial decomposition.

It is a clear yellow liquid, of density 1.327 at 25° C.; its vapour is much more pungent than that of the trisulphide and more severely attacks the eyes and mucous membranes. The melting-point is -89.6° C., and the boiling-point 70.7° C. It is soluble in the same solvents as the trisulphide.

Towards alkalis the disulphide is much less stable than the trisulphide and decomposes almost explosively in an untreated glass flask. Distilled water induces rapid decomposition, whilst contact with alkali causes explosive formation of hydrogen sulphide. When placed on paper or on the skin, rapid decomposition occurs, in the latter case with formation of a white fleck, resembling the effect of hydrogen peroxide. The disulphide resembles the trisulphide in its behaviour with sulphuric acid and with silver oxide; it is more readily inflamed than the trisulphide.

From the composition of hydrogen disulphide and its chemical behaviour, it is natural to regard it as the analogue of hydrogen peroxide, and as having therefore the structure H•S•S•H. It is probable that the higher polysulphides are of a similar " chain " type, that is, of constitutions H•S•S•S•H, H•S•S•S•S•S•H, and the missing member of this series, H₂S₄, may be present, in addition to hydrogen pentasulphide, in crude hydrogen persulphide.

On the other hand, the suggestion has been made that the poly-sulphides are analogous in structure to the periodides and are to be represented as additive compounds of hydrogen sulphide with sulphur of the general formula H₂S.S_n. In such a case, the additional sulphur atoms would presumably be attached in the manner , etc. This view possesses an advantage in giving a possible reason why hydrogen disulphide should be more stable than the higher poly- sulphides towards rise in temperature, because, of the polysulphides, it alone would be free from a quadrivalent sulphur atom with its four valencies entirely satisfied by other sulphur atoms. Indeed, the change in colour of hydrogen trisulphide with alteration of temperature, as well as the low stability of the trisulphide relative to the disulphide at higher temperatures, have been tentatively referred to the existence of dynamic isomerism between the two structures H•S•S•S•H ⇔ .

The evidence, however, is very slight, and this suggestion, together with that of a structure for hydrogen pentasulphide, making the latter structurally analogous to sulphuric acid, must be regarded as requiring experimental confirmation before serious consideration can be claimed.

An alternative method of representing the formulae of the polysulphides has been proposed, namely



and this shares with the earlier H₂S.S_n formulae the advantage of indicating a possible reason for the relatively greater stability of hydrogen disulphide at higher temperatures. This type of constitution is favoured also by the behaviour of the polysulphides of sodium and potassium when heated in hydrogen. Sulphur is readily removed from the higher sulphides, but the disulphides of both metals are very stable compounds from which sulphur can only be removed with difficulty at 700° to 800° C. The evidence supplied by the organic polysulphides corresponding with hydrogen disulphide, trisulphide, etc., appears, however, to be strongly in favour of the " chain " structure mentioned at the commencement of this discussion. Hydrogen disulphide and trisulphide form additive compounds with aromatic aldehydes such as benzaldehyde, e.g. (C₆H₅CHO)₂.H₂S₂, (C₆H₅CHO)₂.H₂S₃, and the behaviour of these products indicates that the sulphur atoms are present in a continuous chain.1 In the light of our present knowledge, therefore, the most satisfactory method of representing the constitution of the hydrogen polysulphides is by means of the "sulphur chain" formulae.