Reductive selenation

Selenolates such as Na2Se, NaSeH, PhSeNa, etc., and tellurolates such as NapTe, NaTeH, PhTeNa, etc., are excellent nucleophiles and can reduce a variety of functional groups by nucleophilic attack or single electron-transfer. On treatment with alkali metal selenolates (or amine salts of HpSe and PhSeH), reduction or reductive selenation of ketones and aldehydes, C=C reduction of a,/l-unsaturated compounds, and reduction of nitrogen compounds such as nitro compounds occur successfully [118, 176]. Compared with these selenolate anions, the corresponding tellurium compounds are highly reactive not only toward the same substrates but also toward halo compounds such as a-bromo ketones and vic-dibromoalkanes [46, 52, 177]. 

Phytoremediation is also being developed for dealing with soils contaminated with high levels of selenium in California again B.juncea seems to be particularly effective in accumulating the contaminant from soil, and all plants tested were more effective at removing selenate than selenite (92). This is an interesting contrast to bacterial systems, where selenite reduction is more commonly found than selenate reduction. 

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. 

Comprehensive accounts of the various gravimetric, polarographic, spectrophotometric, and neutron activation analytical methods have been pubHshed (1,2,5,17,19,65—67). Sampling and analysis of biological materials and organic compounds is treated in References 60 and 68. Many analytical methods depend on the conversion of selenium in the sample to selenous acid, H2Se02, and reduction to elemental selenium when a gravimetric deterrnination is desired. 

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and teUurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 W with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxylamine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and deterrnination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

Let us add here that despite the general similarities of selenium and sulfur in their chemical properties, the chemistry of selenium differs from that of sulfur in two important aspects their oxoanions are not similarly reduced, and their hydrides have different acid strengths. For example, Se(-HlV) tends to undergo reduction to Se(-II), whereas S(-hIV) tends to undergo oxidation. This difference is evidenced by the ability of selenous acid to oxidize sulftirous acid ... 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

Skyllas-Kazacos M, Miller B (1980) Studies in selenous acid reduction and CdSe film deposition. J Electrochem Soc 127 869-873... 

The authors [35] emphasize that their result regarding the first HgS monolayer, which involves reversible underpotential adsorption, suggests that nucleation cannot be considered as a universal mechanism for the formation of anodic films. Analogous conclusions have been inferred for cathodic HgSe films electrodeposited on mercury electrode by the reduction of selenous acid [37] the first monolayer appeared to be reversibly adsorbed, while formation of the following two layers was preceded by nucleation. 

The co-reduction of copper and selenium is considered as an exception to Kroger s theory. Current-potential curves in the literature show that deposition of copper is rather compulsory to make the deposition of selenium possible. In fact, although the standard potential for Se(IV) reduction is more positive than that of copper (0.741 and 0.340 V vs. SHE, for selenous acid and cupric ion, respectively), it turns out that Se(IV) alone is reduced at more negative potentials than Cu(II). In the presence of copper, the order is reversed. 

Maiers DT, PL Wichlacz, DL Thompson, DF Bruhn (1988) Selenate reduction by bacteria from a selenium-rich environment. Appl Environ Microbiol 54 2591-2593. 

Ridley H, CA Watts, DJ Richardson, CS Butler (2006) Resolution of distinct membrane-bound enzymes from Enterobacter cloacae SKLDla-1 that are responsible for selective reduction of nitrate and selenate anions. Appl Environ Microbiol 12 5173-5180. 

Sabaty M, C Avazeri, D Pignol, A Vermeglio (2001) Characterization of the reduction of selenate and tellurite by nitrate reductases. Appl Environ Microbiol 67 5122-5126. 

Selenate is the most mobile form of Se. Selenium becomes biologically unavailable by reduction to elemental Se or by formation of metal selenides or Se-sulfides. Inorganic Se compounds can be converted to volatile organic Se such as dimethyl selenide or dimethyl diselenide by... 

Nanosized selenium particles were deposited onto Ti02 by the photocatalytic reduction of selenate (Se(VI) and selenite (Se(IV) ions) [317], The deposition of Se particles on TiOz was only observed in... 

Reduction of sulfate, nitrate, selenate and chromate Analogous isotopic behavior... 

The body of research on isotopic fractionation induced by sulfate and nitrate reduction provides insight into selenate, selenite and chromate reduction. For sulfate and nitrate oxyanions, reduction is generally microbially mediated, is irreversible, and involves a fairly large but variable isotopic fractionation. As described below, Se and Cr oxyanion reduction follows suit, though abiotic reactions may have a greater role in some transformations. 

Because this reaction must involve two steps, diffusion of selenate into the interlayer spaces of the green rust followed by electron transfer from Fe(ll) green rust, Johnson and Bullen (2003) interpreted this result using a two-step model similar to that discussed above. The diffusion step presumably has very little isotopic fractionation associated with it. Step 2 might be expected to involve a kinetic isotope effect similar to that observed in the HCl reduction experiments. As is discussed above, if the diffusion step is partially rate-limiting, the isotopic fractionation for the overall process should be less than the kinetic isotope effect occurring at the reduction step. This appears to be the case, as the ese(vi)-se(iv) value of 7.4%o is somewhat smaller than that observed for reduction by strong HCl (12%o). 

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