Tellurium reactions

Correlate reaction tellurium-mediated resolution of racemic allyl alcohols... 

Since in the described reactions tellurium is regenerated and sodium hydrogen tellnride is prepared from the element and sodium borohydride, a catalytic procedure is also effective, in which only catalytic amounts of preformed reagent are employed. ... 

Hydrogenation Catalysts. The key to catalytic hydrogenation is the catalyst, which promotes a reaction which otherwise would occur too slowly to be useful. Catalysts for the hydrogenation of nitro compounds and nitriles are generally based on one or more of the group VIII metals. The metals most commonly used are cobalt, nickel, palladium, platinum, rhodium, and mthenium, but others, including copper (16), iron (17), and tellurium... 

In many reactions, selenium is an oxidant as well as a reductant. Strong oxidants convert selenium dioxide and its derivatives to the hexavalent state. Although hexavalent selenium compounds are oxidants, these are less active and difficult to reduce. Selenium salts resemble the corresponding sulfur and tellurium salts in behavior. 

Sulfation Roasting. Acid roasting technology (Fig. 2) rehes on differences in the volatiUty of the tetravalent oxides of selenium and tellurium at roasting temperatures of 500—600°C to selectively volatilise selenium from slimes. Acid roasting uses sulfuric acid as the oxidant for the conversion of selenium/selenides and tellurium/teUurides to their respective tetravalent oxides. Typical oxidation reactions are as foUow ... 

It is apparent from these equations that significant quantities of sulfur dioxide are generated. For selenium, the reaction shown for oxidation of elemental selenium reverses itself at the lower temperatures employed for water scmbbing, thus regenerating sulfuric acid. The tellurium dioxide remains in the sulfated slimes. 

Losses of selenium and tellurium from the solution are negligible, provided the reactor is equipped with a reflux condenser. The wet chlorination is easily controlled. The reaction is rapid, allowing fast turnover of the precious metals in the slimes and yielding all the selenium and tellurium in soluble form. 

Tellurium and many other impurities remain undissolved. The solution is filtered and cooled to reverse the reaction and to deposit soHd selenium. Oeselenized liquor is recycled to the dissolution step. 

Tellurium diethyldithiocarbamate [20941 -65-5] [(C2H3)2NC(S)S]4Te, is made by the reaction of diethylarnine, carbon disulfide, and tellurium dioxide in an alcohoHc solution. 

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

Ghalcogenides. The reactions of pure zirconium turnings with threefold quantities of elemental sulfur, selenium, or tellurium give ZrS ... 

Although it is only slowly oxidized in moist air at ambient temperature, cadmium forms a fume of brown-colored cadmium oxide [1306-19-0] CdO, when heated in air. Other elements which react readily with cadmium metal upon heating include the halogens, phosphoms, selenium, sulfur, and tellurium. The standard reduction potential for the reaction... 

Caution Because tellurium compounds have toxic effects similar to those of arsenic compounds care should be taken not to bring tellurium tetrachloride and its reaction products into contact with the skin. Avoid breathing fumes and dust of tellurium compounds. In addition, hydrogen chloride is evolved in Step A, and pyrophoric Raney nickel is used in Step B. Therefore all manipulations described in this procedure must be carried out in an efficient fume hood. 

Tellurium(VI)-nitrogen bonds can be generated by the reaction of hexamethyldisilazane with tellurium hexafluoride (Eq. 2.12). The product (Mc3SiNH)TeE5 is a useful precursor for a variety of NTeEs compounds. By contrast, SEe is inert towards Si-N reagents. 

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