Thiocyanogen, (CNS)₂

Thiocyanogen, (CNS)₂, was first obtained by Soderback by the action of bromine or iodine on a suspension of the thiocyanate of silver, lead, cadmium, mercury, zinc, thallium or copper in carbon disulphide:

2MSCN + Br₂ = 2MBr + (CNS)₂.

It may also be prepared by electrolysis of the alkali thiocyanates in aqueous or alcoholic solution, using a platinum gauze anode and a silver cathode. On evaporation under reduced pressure, thiocyanogen is obtained as a viscous oil, solidifying at -70° C.

A usual method for the preparation of a solution of thiocyanogen is to treat lead thiocyanate with a dry ethereal solution of bromine cooled in ice.

When a solution of thiocyanogen in carbon disulphide is cooled to -70° C., the thiocyanogen is obtained in cruciform aggregates of almost colourless crystals, melting at -2° to -3° C. On warming to ordinary temperatures the thiocyanogen becomes reddish-brown in colour and more viscous; finally a brick-red amorphous solid is obtained. Thiocyanogen is very readily soluble in ethyl alcohol and ether, slowly soluble in carbon disulphide and carbon tetrachloride.

In many of its reactions, and in its molecular formula, thiocyanogen shows a close analogy with the halogens. Its molecular weight has been determined by the cryoscopic method, allowing a known weight of bromine to react with lead thiocyanate in the presence of bromoform,

Pb(CNS)₂ + Br₂ = PbBr₂ + (CNS)₂,

and measuring the depression of the freezing-point thus obtained. The result obtained is in agreement with that required by the molecular formula (CNS)₂.

When thiocyanogen is treated with chlorides or bromides no appreciable effect is produced. It liberates iodine from aqueous or alcoholic solutions of the iodides of cadmium, lead, silver and mercury. When treated with iron powder, or mercury, the corresponding thiocyanates are formed. Water interacts with thiocyanogen to form thiocyanic acid, hydrogen cyanide and sulphuric acid.

The reactions of thiocyanogen may roughly be divided into two types: . (1) Reactions in which the radical combines directly with metals to form the corresponding thiocyanates, and with cuprous thiocyanate to form the cupric salt. (2) Reactions in which a substitution is effected; for example, with aniline, dimethylaniline and phenol, the corresponding p-thioeyano-derivatives and thiocyanic acid are formed.

According to Kerstein and Hoffmann a further analogy between thiocyanogen and iodine lies in the formation of trithiocyanates by the union of thiocyanogen with thiocyanates. These trithiocyanates behave like free thiocyanogen, except for their lesser sensitiveness towards water.

Thiocyanogen in solution in chloroform may react with chlorine in three distinct ways yielding (1) thiocyanogen monochloride, SCNCl, (2) sulphur chloride and cyanuric chloride, and (3) thiocyanogen trichloride, SCNCl₃.

Solutions of thiocyanogen in organic solvents can be titrated accurately by agitation with at least twice the equivalent quantity of potassium iodide and determination of the liberated iodine.

The application of thiocyanogen in volumetric analysis is restricted by the necessity of using anhydrous solvents and dry vessels, to avoid hydrolysis. With a sufficient excess of sodium thiosulphate or hydrogen sulphide, respectively, thiocyanogen reacts quantitatively according to the equations:

(CNS)₂ + 2Na₂S₂O₃ = 2NaSCN + Na₂S₄O₆
and
(CNS)₂ + S'' = 2SCN' + S.