Selenium Monochloride, Se₂Cl₂

Selenium Monochloride, Se₂Cl₂ is most easily obtained by the action of chlorine on heated selenium, but it is always accompanied by a certain quantity of the more stable tetrachloride which, however, is less volatile. It may be prepared by saturating with chlorine a suspension of selenium or a selenium mineral in carbon tetrachloride. The selenium monochloride is soluble in carbon tetrachloride, whilst the chlorides of other elements present are insoluble. The monochloride is therefore obtained by evaporation of the solvent after filtration.

The chlorides of certain non-metals such as phosphorus have a similar action on selenium, possibly on account of previous dissociation with formation of chlorine.

In some commercial acids selenium is present in the form of the monochloride, produced when seleniferous sulphuric acid is heated with salt. On the addition of ferric chloride, the monochloride gradually separates.

A solution of selenium in fuming sulphuric acid gives a gradual separation of the monochloride on the addition of hydrogen chloride.

Selenium monochloride is a clear reddish-brown liquid of density (D^25°_4°) 2.7741, and can be condensed to a solid of melting-point -85° C. Its odour somewhat resembles that of sulphur monochloride. The liquid has a refractive index of 1.59617 at 25° C., and its specific conductance is 5.12×10^-6 mho. On vaporisation it undergoes partial dissociation. Boiling commences at 145° C., the yellowish-brown vapour containing the tetrachloride and a residue of selenium being obtained in the retort:

2Se₂Cl₂ SeCl₄ + 3Se.

The monoehloride is soluble in various inert organic liquids, more particularly in benzene, chloroform, carbon tetrachloride and carbon disulphide, without undergoing chemical change. It is an exothermic compound, its heat of formation from gaseous chlorine and the amorphous modification of selenium being 22.1 Cals. Water causes a gradual decomposition of the chloride, selenium dioxide and selenium being formed:

2Se₂Cl₂ + 2H₂O = SeO₂ + 3Se + 4HCl.

Selenium monochloride behaves as a strong chlorinating agent towards metals, metallic selenides and hydrocarbons. Phosphorus displaces selenium from the chloride with formation of phosphorus trichloride. Chlorine converts it into the tetrachloride.

The action of liquid ammonia on selenium monochloride is exceedingly vigorous and results in the deposition of red selenium. The monochloride in ether solution, however, yields with liquid ammonia a pale brown amorphous compound of formula Se₂NCl.

The constitution of selenium monochloride is uncertain; it may be represented by

or Cl-Se-Se-Cl; that the molecular weight is in accordance with this formula has been demonstrated by cryoscopic investigation in solution in ethylene dibromide.