Tellurium compounds oxidation

Preparation and Properties of Tellurium(IV) Oxide. Tellurium compounds are poisonous Run experiments with tellurium in a fume cupboard Even negligible amounts of tellurium and its compounds getting into a human organism cause an odour of garlic that does not vanish for a long time. 

Te2Fio, and oxide fluorides, e.g. TeFjOTeFs, are also formed during the fluor-ination of tellurium oxides, tellurium, organic derivatives Tellurium forms organic derivatives in the +2 and +4 slates. The +2 compounds are similar to divalent sulphur derivatives although less stable. Tellurium(IV) derivatives are comparatively unstable. 

Organic tellurium compounds and siliceous materials, ie, rock, ore, or concentrates, are fused with mixtures of sodium carbonate and alkaline oxidants, ie, sodium peroxide, potassium nitrate, or potassium persulfate. For volatile compounds, this fusion is performed in a bomb or a closed-system microwave digestion vessel. An oxidising fusion usually converts tellurium into Te(VI) rather than Te(IV). 

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

The mechanism of tellurium resistance has been investigated using genetic manipulation similar to that of Se (see above) and cellular oxidant capacity apparently plays an important role.144,206 A few tellurite determinants - both chromosomal and plasmid encoded - have been identified in bacte-ria.113,147 192 207 208 Recent studies have focused on the role of methyltransf-erases in Te resistance. Liu et a/.111 determined that the E. coli gene tehB uses S-adenosyl methionine and a methyltransferase in tellurite detoxification, but while no methylated tellurium compounds (see below) were observed, a loss of tellurite was observed in tellurite-amended cultures and Te complexation was inferred.191... 

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

Tellurium(IV), 24 414 oxidation of, 24 415 Tellurium (Te), 24 404-433. See also HgCdTe photodiodes Tellurium compounds... 

Two-electron oxidations and reductions of selenium and tellurium compounds 80... 

The morning session was devoted to a general explanation of the areas of application in studying magnetic properties, oxidation states, compounds, and metal structure. In the afternoon, reviews of the Mossbauer investigations of iron, tin, iodine, tellurium, and some of the rare earth elements were presented. The meeting concluded with a discussion on the future of Mossbauer Spectroscopy in which an interested audience participated. 

Tellurium forms inorganic compounds very similar to those of sulfur and selenium. The most important tellurium compounds are the tellurides, halides, oxides, and oxyacids (5). Techniques and methods of preparation are given in the literature (51,52). The chemical relations of tellurium compounds are illustrated in Figure 2 (53). 

Analogous oxides are known containing one atom of sulphur replaced by selenium or tellurium. The tellurium compound, tellurium sulphoxide, STeOa, decomposes with production of tellurium monoxide, TeO, when gradually heated in a vacuum to 225° C.8 Attempts to produce sulphur monoxide similarly from sulphur sesquioxide have not been successful. 

In the processes described in which the tellurium is precipitated in the elementary form, it is generally assumed (see p. 365) that the error due to oxidation of the precipitate is practically negligible under the conditions of the experiment. Browning and Flint,6 however, maintain that the results are liable to be inaccurate owing to this oxidation. Tellurium dioxide, on the other hand, is unaffected by the air, is anhydrous, non-hygroscopic and easily obtained in the pure condition, and Browning and Flint base a method for the estimation of tellurium on precipitation as dioxide. The tellurium compound is precipitated from a faintly acid solution by means of ammonia, the acidity being restored by the cautious addition of acetic acid. The mixture is heated for some time to render the precipitate crystalline. The method is applicable to the separation of tellurium from selenium.7... 

The tendency of the chalcogens to show increasing metallic character as one moves down the periodic table is quite marked for polonium in fact, it resembles lead more than it does tellurium Its compounds have a more ionic character in its lower oxidation states than do the tellurium compounds. The stability of the 6+ state is low. the existence of polonate(VI) ion being doubtful. The common oxidation states of the element are 2+ and 4+. 

The selectivity in the formation of 1,4-diacetoxy-2-butene (1,4-DAB) is considerably enhanced when tellurium compounds are used as cocatalysts. Thus a heterogeneous catalyst, prepared by impregnation of Pd(N03)2 and Te02 dissolved in HN03 over active charcoal (Pd/Te = 10), can be used for the oxidation of butadiene (by 02 in AcOH at 90 °C) to 63% trans-l,4-DAB, 25% cis-1,4-DAB and 12% 3,4-diacetoxy-l-butene. Conventional soluble catalysts such as Pd(OAc)2/Li(OAc) are much less selective in the formation of 1,4-DAB 429 The gas-phase 1,4-diacetoxylation of butadiene in the presence of Pd-Te catalysts is currently being industrially developed by Mitsubishi and BASF 430... 

Organotellurols 1, the tellurium analogs of alcohols, constitute a class of organic tellurium compounds, which have to be prepared and used in situ due to their high succeptibility to oxidation. The participation of this class of compounds in several reactions has been proposed,5-12 but their real existence has never been demonstrated in such processes. A few tellurols exhibiting special structural features could be isolated and characterized (e.g., compound 10, Figure 2).14... 

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