Redox reactions selenium

As discussed earlier the whole process is a redox reaction. Selenium is reduced using sodium borohydride to give selenide ions. In the above reaction, the metal ion reacts with the polymer (PVP or PVA) solution to form the polymer-metal ion solution. Addition of the selenide ion solution to the polymer-metal ion solutions resulted in instantaneous change in the colour of the solutions from colourless to orange (PVA) and orange red (PVP). This indicates the formation of CdSe nanoparticles. The addition of the selenide solution to the polymer - metal ion solution resulted in gradual release of selenide ion (Se -) upon hydrolytic decomposition in alkaline media (equation 4). The released selenide ions then react with metal ion to form seed particles (nucleation). 

We have reported a simple, green, bench top, economical and environmentally benign room temperature synthesis of MSe (M=Cd or Zn) nanoparticles using starch, PVA and PVP as passivating agents. The whole process is a redox reaction with selenium acting as the oxidant and MSe as the reduction product. An entire "green" chemistry was explored in this synthetic procedure and it is reproducible. The optical spectroscopy showed that all the particles are blue shifted from the bulk band gap clearly due to quantum confinement. Starch capped CdSe nanoparticles showed the presence of monodispersed spherical... 

Multicomponent reactions (MCRs) have been known to produce highly complex and diverse structures [76]. There is a considerable interest in the application of new multicomponent reactions to access biologically relevant molecules [77,78] and natural products [79]. A recent report has disclosed multicomponent Passerini and Ugi reactions to produce, rapid and efficiently, a library of redox-active selenium and tellurium compounds [80]. The compounds showed promising cytotoxicity against several cancer cell lines. 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

Redox reactions of selenium could now be measured. A convenient example was the oxidation of selenium bound to sulphur in N-acetylcysteine-selenotrisulphide. Upon... 

As an application of selenium chemistry, reduction of aryl ketones with CO and H2O in the presences of selenium and DBU has been carried out. The reduction, indicated in Scheme 19, can be considered as a redox reaction by CO and H2O. This reaction proceeds in high yield with only a catalytic amount of selenium. Although the reaction mechanism has not been clarified, this reaction probably involves an or-ganoselenium hydride intermediate (60), which is known to give the corresponding hydrocarbon under the reaction conditions." ... 

Selenium is commonly found in oxidation states II, IV, and VI, and as an element. Se(VI) is much more stable than Se(IV). The reaction chemistry of selenium is mainly that of selenide (Se ), selenite (SeOa ), and selenate (Se04 ). Selenious acid is a weak acid with a p a of 2.6 and 8.3. Selenious acid and selenite are much stronger oxidants than sulfurous acid and sulfite. Thus, many of the characteristic reactions are related to redox reactions, in which Se(IV) can be reduced to elemental selenium. The acid strength of selenic acid (H2Se04) is similar to sulfuric acid. 

Tokunaga TK, Brown GE Jr., Pickering IJ, Sutton SR, Bajt S (1997) Selenium redox reactions and transport between ponded waters and sediments. Environ Sci Technol 31 1419-1425... 

This is an area which has only very recently attracted intensive study. In fact, as recently pointed out by Stadtman detailed studies of the specific biochemical role of selenium at the enzyme level dates only from 1972. However, research is progressing rapidly. To put this into perspective, a 1979 report stated that there are at least three, and possibly four , selenoproteins. A 1980 article listed eight known selenoproteins Table 2 lists these selenoproteins and their source. Interestingly, the form of selenium which has been determined in all animal and bacterial proteins examined thus far appears to be selenocysteine. Both bacterial and mammalian enzymes apparently are involved in redox reactions (an exception may be thiolase see below). Selenomethionine, as mentioned previously, is a predominant form of selenium in many grains and grasses, but it has not been detected in mammalian enzymes. 

Inorganic anions such as iodide, iodate, nitrate, and chlorate have been determined on the basis of redox reactions. Iodides have been determined by their reaction with chromium(vi) in an acidic medium. The unreacted Cr is then extracted into MIBK from a 3 M HCl solution. The iodide concentration is quantitatively related to the increase of the Cr atomic signal in aqueous solution or the decrease of the Cr signal in the organic phase. Selenium(iv) is reduced by iodide to elemental selenium. The atomic absorption signal of selenium is then measured in the selenium precipitate. 

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