Metals selenium

This chapter is an attempt to present the important results of studies of the synthesis, reactivity, and physicochemical properties of this series of compounds. The subject was surveyed by Bulka (3) in 1963 and by Klayman and Gunther (4) in 1973. Unlike the oxazoles and thiazoles. there are few convenient preparative routes to the selenazoles. Furthermore, the selenium intermediates are difficult to synthesize and are often extremely toxic selenoamides tend to decompose rapidly depositing metallic selenium. This inconvenience can be alleviated by choice of suitable reaction conditions. Finally, the use of selenium compounds in preparative reactions is often complicated by the fragility of the cycle and the deposition of metallic selenium. 

DiaminoseIenazole was obtained by the action of selenourea on chloracetonitrile (15). Its salts are fairly unstable and decompose to give metallic selenium. All attempts to obtain the free base or 2,4-dihydroxyselenazole from 2,4-diaminoseIenazole hydrochloride were unsuccessful and led to decomposition of the heterocycle. 

Alkyl selenazoles have been sulfonated. Thus 2,4-dimethylselenazole with fuming sulfuric acid (5% SO3) at 100°C gives 2.4-dimethy -5-sulfonylselenazole [m.p. 238 C (decomp.) (26)], which is relatively unstable. decomposing to give metallic selenium. 

Selenium occurs in the slimes as intermetallic compounds such as copper silver selenide [12040-91 -4], CuAgSe disilver selenide [1302-09-6], Ag2Se and Cu2 Se [20405-64-5], where x < 1. The primary purpose of slimes treatment is the recovery of the precious metals gold, silver, platinum, palladium, and rhodium. The recovery of selenium is a secondary concern. Because of the complexity and variabiUty of slimes composition throughout the world, a number of processes have been developed to recover both the precious metals and selenium. More recently, the emphasis has switched to the development of processes which result in early recovery of the higher value precious metals. Selenium and tellurium are released in the later stages. Processes in use at the primary copper refineries are described in detail elsewhere (25—44). 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

The mud or slime is coUected from the bottom of the electrolytic ceUs and pumped to the sUver refinery, where it is processed for recovery of copper, precious metals, selenium, and, in many cases, teUurium. The anode slime contains 2—20% selenium as copper and sUver selenides, whereas gold exists as the metal and in combination with teUurium. A flow diagram is shown in Figure 8. 

Determination of copper as copper(I) thiocyanate Discussion. This is an excellent method, since most thiocyanates of other metals are soluble. Separation may thus be effected from bismuth, cadmium, arsenic, antimony, tin, iron, nickel, cobalt, manganese, and zinc. The addition of 2-3 g of tartaric acid is desirable for the prevention of hydrolysis when bismuth, antimony, or tin is present. Excessive amounts of ammonium salts or of the thiocyanate precipitant should be absent, as should also oxidising agents the solution should only be slightly acidic, since the solubility of the precipitate increases with decreasing pH. Lead, mercury, the precious metals, selenium, and tellurium interfere and contaminate the precipitate. 

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

The Selocide spray residue remaining on the fruit is red metallic selenium (5). 

For the preparation of secondary and tertiary selenoamides, several methods have been used reaction of lithium alkyneselenolates (obtained using metallic selenium) with amines, aromatic diselenoic acid Se-methyl esters with amines or dihalomethane with elemental selenium, NaH and amines (Scheme 83).258 266... 

Bebbington, G.N., N.J. Mackay, R. Chvojka, R.J. Williams, A. Dunn, and E.H. Auty. 1977. Heavy metals, selenium and arsenic in nine species of Australian commercial fish. Austral. Jour. Mar. Freshwater Res. 28 277-286. 

Bis(4-toluenesulphonyl)selenodiimide (TosN=Se=NTos), prepared from Chloramine T and metallic selenium, reacts with olefins by insertion at the more substituted allylic... 

Although a toxic metal, selenium in trace amounts is a nutritional element. Trace amounts added to cattle food are effective against muscular dystrophy in sheep and cattle. 

The compound is toxic by ingestion. Symptoms of the poisoning effects of selenium dioxide are similar to those of selenium metal. Selenium dioxide vapors are highly irritating to eyes, nose and respiratory tract. 

In this reaction, the initially introduced Se02, insoluble in organic solvent, is converted to soluble SeCO by CO (30 atm) under reflux in toluene, which is the active catalyst for the reductive carbonylation of nitrobenzene or nitropyridine. The metallic selenium (insoluble) is also converted to SeO under CO pressure. After completion of the reaction, soluble selenium catalyst solution was readily recovered by simple filtration and reused. The recovered catalyst solution was used for five cycles without loss of activity. 

The selenium filtered from the reaction mixture is refluxed with 31. of 95% ethanol for 1 hour, which converts the red amorphous form to the gray hexagonal form and frees it of organic matter. This metallic selenium is removed by filtration, washed with water, and converted to the dioxide by the method of Baker and Maxson.2... 

Solid Allotropes.—Both amorphous and crystalline varieties of selenium occur. Amorphous selenium is best known as the vitreous and the finely divided brick-red forms, which are frequently described as two distinct allotropes they are, however, identical. The crystalline allotropes include several monoclinic varieties, red to brown in colour, as well as the so-called metallic selenium. 

Monoclinic selenium has a density of 4-44 3 at 0° C. It is only sparingly soluble in carbon disulphide, giving a red solution. When heated rapidly it melts at 170° to 180° C., but transformation into metallic selenium commences to occur slowly near 120° C.4 The liquid obtained by rapid heating passes into the vitreous condition when cooled. 

Electric cables treated with a thin layer of metallic selenium are flame-proof to a remarkable degree, thus minimising the risk of fire through short-circuiting of heavy currents. The cotton or rubber used for covering the wire may itself be treated with the selenium and so rendered non-inflammable flame-proof switchboards, etc., may also be constructed. 

Selenium dioxide is an exothermic compound, the heat of formation being 57-2 Calories per gram-molecule, referred to vitreous selenium, whilst the value is somewhat less for the monoclinic element and less still for metallic selenium.1... 

The selenites are readily attacked by micro-organisms with the formation of a red substance, probably metallic selenium.13 The... 

Crystalline Tellurium. — Molten tellurium solidifies to a brittle, silvery, crystalline mass, which is easily powdered. The crystalline modification can also be obtained by sublimation of the element or by its slow formation, for example in the gradual decomposition of hydrogen telluride5 or in the slow atmospheric oxidation of an aqueous solution of an alkali telluride.6 When obtained of appreciable size the crystals are generally found to be prismatic, of the trigonal system, and isomorphous with metallic selenium (a c=l 1-3298 a=86-8°).7... 

Culvenor and co-workera also examined the action of potassium wlenocyanate with oyctohcxene oxido and stilbene oxide.8e < T Cvolohexene and Btilbene are regenerated in high yield, accompanied by liberation of metallic selenium (Eqs. 416 and 416). 

Di-o-tolyl selenide, Se(CH3.C6H4)2.6—This is prepared by the action of selenium on magnesium o-tolyl bromide, and is isolated during the preparation of o-tolylselenoglycollic acid after extraction of the ether solution of selenomagnesium bromide with aqueous potassium hydroxide. Repeated crystallisation from absolute alcohol gives flat, colourless, shining plates, M.pt. 64° C. This selenide also occurs when mercury di-o-tolyl is heated with metallic selenium 7... 

Benzyl a-naphthyl selenide, (C6H5.CH2)Se(C10H7).2—This selenide is produced when the complex obtained from magnesium a-naphthyl bromide and metallic selenium is treated with benzyl chloride. It forms small colourless prisms from alcohol, M.pt. 68° to 69° C. It yields a picrate, which separates from ether in orange-red needles, M.pt. 118° C. 

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