Tellurium production

Tellurium dioxide in its orthorhombic form occurs in nature as mineral tellurite. It is mined from natural deposits. Also, tellurium dioxide is produced as an intermediate during recovery of tellurium metal from anode shmes of electrolytic copper refining (See Tellurium, Production). The dioxide also is prepared by treating tellurium metal with hot nitric acid to form 2Te02 HNO3. The product then is heated to drive off nitric acid. 

Alternatively, if tellurium dioxide is the product desired, the sodium tellurite solution can be neutralized in a controlled fashion with sulfuric acid. As the pH is lowered, precipitates containing impurities such as lead and silica that form are filtered off. At pH 5.6 the solubility of tellurous acid reaches a minimum and essentially all of the tellurium precipitates (>98%). After filtration and drying, commercial tellurium dioxide is obtained. A diagram for the process of detellurizing of slimes and recovering tellurium products is shown in Figure 1. 

Diorgano ditellurium compounds and diorgano mercury reagents react also to give diorgano tellurium products. In these reactions both halves of a di tellurium substrate are converted to the desired product. 

Unsymmetrical dialkyl tellurium derivatives were prepared by mixing an aqueous disodium telluride solution with equimolar amounts of two different alkyl halides. All three possible dialkyl tellurium products are formed. The unsymmetrical dialkyl tellurium is the predominant species. It can be separated from the symmetrical compounds by chromatography1. This one-pot procedure takes less time to complete than the alternative route employing alkyl tellurolates (p. 387) and was used to prepare unsymmetrical dialkyl telluriums containing radioactive tellurium. Sequential addition of two alkyl halides produced only symmetrical dialkyl telluriums. 

The following diaryl tellurium products were prepared in this manner3 ... 

The reactions between Grignard reagents and tellurium dihalides frequently reported in the older literature (Vol. IX, p. 1051) produced mixtures of diaryl tellurium products and other organic tellurium compounds that were difficult to separate. Tellurium dihalides are rather unstable compounds with a strong tendency to disproportionate to tellurium and tellurium tetrahalides. The Grignard reagents, which had to be used in excess of the stoichiometri-cally required amounts, then reacted with the tellurium (p. 172) and the tellurium tetrahalide and probably not with the tellurium dihalide. 

Diaryl tellurium compounds can be obtained in moderate yields from potassium tellurocyanate and arenediazonium tetrafluoroborates in DMSO at room temperature. The potassium tellurocyanate is obtained from tellurium and potassium cyanide in DMSO. Aryl tellurium cyanides are probably formed as intermediates that disproportionate to the diaryl tellurium products and tellurium dicyanide2. 

Arenetellurolates, ethenetellurolates, and alkanetellurolates prepared by reduction of diorgano ditellurium compounds with sodium borohydride in ethanol, THF/ethanol, or DMSO add to acetylenes in regioselective and iran.y-stereoselcctive reactions to produce aryl ethenyl tellurium products either predominantly or exclusively as (Z)-isomers. The yields are almost always higher than 70%. In reactions with acetylenic aldehydes, ketones, carboxylic acids, and esters the arenetellurolate becomes bonded to the carbon atom in a [i-position to the carbonyl group. 

Oxiranes4,5 and oxiranocycloalkanes5 8 reacted with sodium benzenetellurolate to produce the 2-hydroxyalkyl or trans-2-hydroxycycloalky 1 phenyl tellurium products. 

Arenetellurolates, prepared from diaryl ditellurium compounds via reduction with sodium borohydride in organic media, interact with arenediazonium chlorides2 and arenediazonium tetrafluoroborates3 to produce unsymmetrical diaryl tellurium products in yields below 50%. The arenediazonium chlorides were added as aqueous solutions to the tellurolates. Arenediazonium tetrafluoroborates were used as solids. 

Oxidation of tellurolates generally produces diorgano ditellurium compounds (p. 264,270). However, the oxidation of ethynetellurolates led to the isolation of diethynyl tellurium products. When sodium ethynetellurolates were hydrolyzed with water in the presence of air, the diethynyl tellurium products were obtained in low yields. Oxidation by iodine in DMSO improved the yield to 70%4. The expected ditellurium compounds were postulated as unstable intermediates. 

The cleavage of an unsymmetrical acyclic ether yields two alkyl phenyl tellurium products. The product distribution is determined by the relative ease of the cleavage of the ether bond by an SN2 process2 ... 

Methyl trimethylsilyl tellurium, generated from lithium methanetellurolate and trimethylsilyl chloride in tetrahydrofuran, reacted with 4-methylbenzenesulfonylazoarenes in acetonitrile at 20° to produce aryl methyl tellurium products in moderate yields2. 

Dialkyl cadmium was employed to convert ort/io-substituted phenyl tellurium halides to unsymmetrical diorgano tellurium products. These conversions were accomplished without affecting the carbonyl groups present in the molecules. 

Diaryl ditellurium compounds will lose one tellurium atom forming diaryl tellurium products when heated above their melting points. This conversion can be affected almost quantitatively by refluxing the diaryl ditellurium with copper powder in toluene or dioxane. 

To facilitate the isolation and purification of the diaryl tellurium products, their conversion to diaryl tellurium dihalides is recommended1. The dihalidcs can then be reduced to the diorgano tellurium products. 

Unsymmetrical aryl ethyl tellurium products were isolated in yields of 50% or less when arenediazonium tetrafluoroborates and diethyl ditellurium reacted in chloroform in the presence of 18-crown-6 under exclusion of light5. This reaction seems to have general applicability and deserves further investigation. 

When bis[4-methoxyphenyl] selenide telluride was heated with 5-methyl-2-(phenyl-iodonio)benzoate, an inseparable mixture of equal amounts of the two isomeric [4 -methoxyphenylseleno)-5-methylphenyl] 4-methoxyphenyl tellurium products was obtained. Molecules with two aryltelluro or two arylseleno groups in the molecule were not detected2. 

Reaction of Diorgano Tellurium Dihalides with Sodium Sulfide General Procedure A 15-fold molar excess of sodium sulfide nonahydrate is mixed with the diorgano tellurium di halide and the mixture is heated at 90-95° on a boiling water bath. When the mixture has become liquid, it is stirred for an additional 10 min. and then cooled. Water is added to dissolve the sulfide and the mixture is filtered to collect the solid diorgano tellurium product. Liquid diorgano tellurium products are extracted into petroleum ether. Products are purified by recrystallization or distillation. 

Disodium telluride condensed with aryl iodides in dimethylformamide on prolonged heating to give diaryl tellurium products in good yields Sodium telluride prepared by the Rongalite method gave better yields than sodium telluride obtained from tellurium and sodium in liquid ammonia or in aprotic solvents, or from tellurium and sodium hydride in hot dimethylformamide -. ... 

The cleavage of an unsymmetrical acyclic ether yields two alkyl phenyl tellurium products. 

Phenyl tellurium bromide and 2-(diphenylphosphano)-phenyl lithium reacted in diethyl ether at — 78° to produce 2-(diphenylphosphano)-phenyl phenyl tellurium in 45% yield . Organo tellurium bromides reacted with vinyl magnesium bromides and vinyl tellurium iodides reacted with organo magnesium bromides to give organo vinyl tellurium products in good yields. 

The presence of an approximately equimolar amount of triethylamine in the reaction mixture binds the methyl iodide formed in the reaction, shifts the equilibrium of the reaction toward the products, and prevents the quatemization of diorgano tellurium products. Whereas pentafluoroethyl iodide requires photochemical stimulation, hep-tafluoroisopropyl iodide does not. ... 

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