Hydrogen selenide, decomposition

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

Many examples of this type of reaction are known the decomposition of arsine the decomposition of phosphine on surfaces of glass, f porcelain, J silica the decomposition of formic acid vapour on a variety of different surfaces— glass, platinum, rhodium, titanium oxide, and others the decomposition of nitrous oxide on the surface of gold Tf the decomposition of sulphuryl chloride on the surface of glass the decomposition of hydrogen iodide on the surface of platinum ff the decomposition of hydrogen selenide on the surface of selenium. J J A general discussion... 

This general reaction can be used for preparing other un-symmetrical thioureas when the desired cyanamides can be obtained. The submitter has also prepared l-methyl-l-(l-naph-thyl)-2-selenourea, m.p. 174-175° with decomposition, and l-ethyl-l-(l-naphthyl)-2-selenourea, m.p. 168-170° with decomposition, by substitution of hydrogen selenide for hydrogen sulfide. 

Eight-membered, and larger, sulfur-selenium rings may be obtained from melts of the elements, by reaction of chlorosulfanes with hydrogen selenide (or vice versa), from selenous acid, or by decomposition of mixtures of 80212 and 8212, although many of these reactions give rise to mixtures... 

This is an irreversible system, and the reaction does not depend on transport from a hotter to a colder zone. Gas phase nucleation is reduced, and the reaction temperature is low (300-500°C). The hydrogen passivation occurs, but still this effect may be partially alleviated by substitution of an alkyl chalcogenide to avoid the hydrogen selenide. Alkylselenium compounds originally were used to avoid the very toxic H2Se. The criteria (e.g., toxicity, decomposition into stable products) for selection of these alternative precursors have been reviewed . 

Hydrogen selenide (H2Se). At 300-400 °C, WSe is formed and, between 500 and 800 °C, the diselenide. In both cases hydrogen is released. Above 850 C, decomposition of the diselenide starts. 

The metabolism of selenium is now fairly well understood. To become incorporated into selenium-specific proteins (e.g., glutathione peroxidase, thioredoxin reductase, iodothyronine 5 -deiodinase) through a cotranslational mechanism requires that selenium be in the form of selenide (Sunde 1990). All forms of selenium can be transformed to selenide, although the rates of transformation vary. For example, selenate is not converted to selenide as readily as selenite. The formation of selenide from selenocysteine requires a specific enzyme, selenocysteine (3-lyase, which catalyzes the decomposition of selenocysteine to alanine and hydrogen selenide. Excess selenium is methylated and exhaled or excreted in the urine in both humans and animals. Further research is required to determine which selenium metabolites or intermediates lead to toxicity. 

Selenides.—The photochemical decomposition of hydrogen selenide has been studiedit was concluded that the only primary step operative in the A > 200.0 nm photolysis is the free-radical mode of decomposition ... 

For its preparation, the commercially available bis(9-borabicyclo-[3.3.1]nonane) (9H-9-BBN)2 is made to react with elemental selenium at about 150°. In larger-scale preparations [with >10 g (9H-9-BBN)2] it is advantageous to add the elemental selenium in small portions in order to avoid a rapid evolution of hydrogen gas. This is in contrast to the procedure used in the similar preparation of the sulfur analog. The monoselenide, as well as, the by-product H2 gas are formed quantitatively. The solid selenide is obtained in essentially pure form. If required, it can be further purified without any decomposition by sublimation. This is in contrast to the... 

Other methods for the synthesis of cyanoacrylate monomers include pyrolysis of 3-alkoxy-2-cyanopropionates (1), transesterification of cyanoacrylates with alcohols (19,20), displacement of monomer from the corresponding anthracene Diels-Alder adduct by treatment with maleic anhydride (7,21), esterification of cyanoacrylic acid or cyanoacryloyl chloride with alcohols (22), and the oxidation of alkyl 2-cyanopropionate phenyl selenides with hydrogen peroxide (23). These alternative methods are particularly useful for the preparation of monomers not readily prepared by thermal decomposition of cyanoacrylate polymer. 

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