Selenium Oxychloride, SeOCl₂

Selenium Oxychloride, SeOCl₂, may be prepared in the following ways:

1. By adding the calculated amount of dry selenium dioxide to selenium tetrachloride suspended in carbon tetrachloride. The oxychloride formed in the cold dissolves in the carbon tetrachloride, from which it may be obtained by distillation. Carbon tetrachloride distils at 76° to 77° C., while the selenium oxychloride distils over at 176.4° C.
2. By the partial hydrolysis of selenium tetrachloride:
   
   SeCl₄ + H₂O → SeOCl₂ + 2HCl.
   
   The tetrachloride may either be used alone or suspended in carbon tetrachloride.
3. When selenium dioxide is brought into contact with dry hydrogen chloride at low temperatures, an amber-coloured liquid is obtained having the formula SeO₂.2HCl. By dehydration of this compound, using phosphorus pentoxide, calcium chloride or other similar dehydrating agent, selenium oxychloride is formed and can be separated by distillation.
   
   This reaction may also be effected by mixing together the selenium dioxide and the dehydrating agent, passing in hydrogen chloride in the cold and then distilling off the oxychloride.
4. When a mixture of selenium dioxide and selenium tetrachloride is heated in a sealed tube, selenium oxychloride is formed:
   
   SeCl₄ + SeO₂ = 2SeOCl₂.
5. Selenium oxychloride is also formed when selenium dioxide is heated with sodium chloride:
   
   2SeO₂ + 2NaCl = SeOCl₂ + Na₂SeO₃.

Selenium oxychloride is a nearly colourless liquid having a boiling- point of 176.4° C. at 726 mm. Its melting-point is 10.8° C. At 20° C. it has a refractive index of 1.6516. The specific conductivity of the dry liquid at 25° C. is 2.0 (±0.3)×10^-5 mho. The oxychloride is, however, extremely hygroscopic, and the presence of traces of water causes a considerable increase in the electrical conductivity. The presence of the merest trace of water can be shown by sealing up a sample of the oxychloride with cobalt carbonate which has been dried at 200° C. for three hours; a blue colour gradually develops if moisture is present.

Selenium oxychloride absorbs all light up to a wave-length of 4050 μμ. It is miscible with chloroform, carbon disulphide and benzene without chemical change. It is also soluble in carbon tetrachloride, but after a time reaction takes place with formation of selenium tetrachloride and carbonyl chloride. At the ordinary temperatures selenium oxychloride is not miscible with the paraffin hydrocarbons, but at higher temperatures (150° to 160° C.) it is completely miscible with higher paraffins such as vaseline.

Selenium oxychloride, when distilled under reduced pressures, has a light straw colour, but when distilled under atmospheric pressure it decomposes slightly and becomes reddish-brown; this decomposition reverses to some extent on cooling.

At temperatures between 265° and 290° C. there is a small amount of dissociation, since the mean value of the molecular weight over this range of temperature is 151.4 (SeOCl₂ = 166.12). The following decompositions occur to a small extent at or near the boiling-point under atmospheric pressure:
2SeOCl₂ ⇔ SeO₂ + SeCl₄,
2SeCl₄ ⇔ Se₂Cl₂ + 3Cl₂,
2Se₂Cl₂ ⇔ SeCl₄ + 3Se;
4SeOCl₂ ⇔ Se₂Cl₂ + 2SeO₂ + 3Cl₂.

The vapour pressure of selenium oxychloride has been determined at temperatures between 84.3° and 117.2° C. It can be expressed by the formula

log P = 5.8503 + 0.000219T – 830.9/(T - 178),

where P = pressure in mm. of mercury and T = absolute temperature.

The dielectric constant of the oxychloride has been determined at 0°, 10° and 20° C. by Schmidt's modification of Drude's method. By this method it has been shown that the liquid has a very high dielectric constant, 46.2±l at 20° C. and 51.00±0.5 at 10° C., and solid selenium oxychloride has the constant 16.8±2 at 0° C.

Sulphur, selenium and tellurium are soluble in the oxychloride in the cold, but on heating, reaction may take place. Sulphur interacts to form selenium monochloride, sulphur monochloride and sulphur dioxide. Tellurium is converted into its tetrachloride. Both red and yellow phosphorus react violently with the oxychloride, forming phosphorus pentoxide and selenium mono- and tetra-chlorides. Carbon, silicon and boron are unattacked.

Selenium oxychloride can be used to show that the essential differences between the adsorptive properties of "retorted" carbon and "activated" carbon are due to the presence of hydrocarbons in the former. After extraction of the hydrocarbons with selenium oxychloride the "retorted" carbon shows the same adsorptive properties as carbon which has been activated by steam at high temperatures.

The majority of the metals and their oxides interact with selenium oxychloride, the vigour of the reaction depending on the metal and on the temperature. Tungsten, tantalum and titanium are, however, only slightly attacked after long treatment at high temperatures, and the oxychloride may be employed as a reagent for the separation of molybdenum and tungsten or of niobium and tantalum. The behaviour of metallic sodium and potassium is interesting. The latter reacts explosively, whilst sodium is unaffected even when the oxychloride is distilled over the metal. Copper reacts with the liquid in a sealed tube according to the equation

3Cu + 4SeOCl₂ = 3CuCl₂ + 2SeO₂ + Se₂Cl₂,

selenides being formed as intermediate products. Silver, nickel and cobalt react similarly, but very slowly; the reaction is more rapid with lead, but there is no evidence of intermediate selenide formation. With the oxides under similar conditions the metal chloride and selenium dioxide result; with red lead or lead dioxide, chlorine also is evolved.

Dry hydrogen sulphide interacts with selenium oxychloride with the formation of yellow selenium sulphide and evolution of hydrogen chloride. There is a development of heat which dissociates the selenium sulphide into sulphur and red selenium. Sulphur dioxide has no action on the hot anhydrous oxychloride, but if water is present there is a deposition of selenium. Sulphur trioxide is soluble in selenium oxychloride, forming a thick solution which is a very powerful solvent for the oxides of the rare earth metals. When the oxychloride is brought into contact with finely divided barium sulphate, the latter is at once peptised and becomes gelatinous in appearance, but when subsequently treated With water the sulphate immediately changes back to the ordinary form.

When the oxychloride is warmed with potassium perdisulphate, chlorine is evolved; if sulphuric acid is present the reaction occurs in the cold. Chlorine is also evolved from selenium oxychloride when warmed with telluric or selenic acid.

Carbon monoxide has no action on selenium oxychloride. Water causes decomposition of the liquid into selenium dioxide and hydrogen chloride. Anhydrous ammonia reacts according to the equation:

6SeOCl₂ + 16NH₃ = 3SeO₂ + 3Se + 2N₂ + 12NH₄Cl.

If the oxychloride is applied in dilute benzene solution some nitrogen selenide or selenium nitride is also formed. With liquid ammonia in the presence of ether in open vessels, the primary product is the compound SeOCl₂.4NH₃, which is decomposed by water into selenium, selenium nitride, ammonium chloride, ammonium selenite and selenious acid; in a sealed tube, selenium nitride is formed in comparatively good yield.

Protein materials such as hair and silk are soluble in the cold oxychloride, but cellulose is not affected.

Selenium oxychloride reacts with unsaturated hydrocarbons and ketones to form well-defined crystalline additive compounds, which, however, are not very stable, being readily decomposed by boiling water to yield their components.

It has been shown that when various rubbers are treated with the oxychloride, their physical properties are changed and they become amorphous powders, insoluble in the ordinary solvents for rubber. According to Frick the behaviour towards selenium oxychloride indicates that there is no difference in empirical composition between the pure rubber hydrocarbon from the Hevea tree and the synthetic rubber made by polymerising isoprene with metallic sodium.

With some metallic chlorides, selenium oxychloride forms additive compounds, for example: TiCl₄.2SeOCl₂, SnCl₄.2SeOCl₂, SbCl₅.2SeOCl₂, FeCl₃.2SeOCl₂, KCl.SeOCl₂, RbCl.SeOCl₂, CaCl₂.3SeOCl₂, MgCl₂.3SeOCl₂. The anhydrous chlorides of lithium, sodium, copper, silver, strontium, barium, zinc, cadmium, mercury, nickel and cobalt apparently do not combine with the oxychloride. Arsenic trichloride is miscible in all proportions.

When distilled with powdered sodium bromide, selenium oxychloride is converted into selenium oxybromide or selenyl bromide, SeOBr₂, a dark red liquid which solidifies to yellow needles.