The Oxides of Selenium

A further series of reactions, under conditions similar to those above, carried out between ZnSe and Se02, ZnS and ZnSe03, ZnS and ZnS04, and ZnSe 

Reactions between ZnO and low concentrations [0.03 (mg Se) 1 ] of gaseous Se02 in a carrier gas have also been studied.  

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Selenium dioxide. The compound is conveniently prepared by the oxidation of selenium with nitric acid, followed by dehydration of the selenious acid thus formed by cautious heating ... 

Little has been reported on the oxidation of selenium functionalities by dioxiranes. One case is the oxidation of selenophenes by DMD " at subambient temperatures. Good yields of either the selenophene 1-oxide or 1,1-dioxide may be achieved, which depends on the amount of DMD used (Scheme 12) . These results are similar to those already presented for the sulfur congeners. ... 

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

COO/WOO] Cooper, J. N., Woods, M., Sullivan, J. C., Deutsch, E., Kinetics of the oxidation of selenium(lV) by neptunium(VlI) in aqueous acidic perchlorate media, Inorg. Chem., 15, (1976), 2862-2864. Cited on page 140. 

Selenium was found to be released during coal mining because of the oxidation of selenium-bearing pyrite (Dreher and Finkelman 1992). 

Selenium(lV) oxide has been prepared by the oxidation of selenium using nitric acid, vapors of fuming nitric acid, oxygen, and oxygen admixed with nitrogen(IV) oxide. When selenium is burned in oxygen a product is obtained that contains appreciable quantities of what is presumed to be red selenium as an impurity. While the combustion of... 

Selenic acid is conveniently prepared by the oxidation of selenium(IV) oxide with 30 per cent hydrogen peroxide. Water and selenious acid are the only possible impurities. The former can be removed effectively by evaporation under reduced pressure the latter is present in small amount. 

This behaviour is explained by moderate values of first ionization energy, electronegativity, electron affinity and atomic radius. Non-metallic oxides are often acidic and react with water. So the oxides of selenium, Se02 and Se03, are expected to react with water to form acidic solutions of H2Se03 and H2Se04. 

Dehydrogenation activities of the oxides of selenium and tellurium were observed by the pulse technique.Thus, the zirconium oxides which are loaded with selenic acid or telluric acid and calcined in air can dehydrogenate 2-propanol to acetone and hexane to benzene.In a typical reaction of 2-propanol, the conversion into acetone decreases continuously after the third pulse, probably owing to a decrease in the amount of oxygen on the catalyst surface. Poisoning experiments with injection of CO2, H2O or butylamine at 523 K before reaction had no effect on the yield of acetone. Thus this dehydrogenation process appears to be an oxidative dehydrogenation. No studies on the acidic or basic character of oxides of selenium and tellurium oxides have been reported. 

Selenium dioxide, SeO, is ery poisonous (cf. p. I47), but is valuable particu larly for the oxidation of methylene ( CHs) groups to carbonyl ( CO) groups. 

CgHjCOCHj + SeOa —> CgHgCOCHO + Se + H O This is one example of the oxidation by selenium dioxide of compounds containing a methylene group adjacent to a carbonyl group to thecorresponding a-ketoaldehyde or a-diketone (see also Section VII,23). 

Conversion of telluiium to the hexavalent form is complete. The extent of selenium oxidation to the hexavalent form is a function of temperature. 

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 ... 

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 oxidation of teUurium(IV) by permanganate as an analytical method has been studied in some detail (26). The sample is dissolved in 1 1 nitric-sulfuric acid mixture addition of potassium bisulfate and repeated fuming with sulfuric acid volatilises the selenium. The tellurite is dissolved in 10 vol % sulfuric acid, followed by threefold dilution with water and titration with potassium permanganate ... 

A number of esters of arsonic and arsinic acids have been prepared. One method involves the oxidation of dialkyl alkylarsonites with selenium dioxide ... 

Recently Swan has employed tetrachloro-o-benzoquinone in the oxidation of the 3,4-dihydro-j8-carbolinium cation 124 to the j8-carbo-linium cation 252. Dehydrogenation with palladium black at 175° or at a higher temperature and with selenium has also been successfully used for the purpose. 

Selenium dioxide may be used for the oxidation of reactive methylene groups to carbonyl groups. An example is the oxidation of cyclohexanone to cyclohexane-1,2-dione (27). In the procedure, the reaction is carried out on camphor to give camphor... 

Since only catalytic quantities of selenium dioxide are required, the danger of handling large quantities of this material (Note 1) is avoided. Furthermore, the problems associated with the formation of selenium and organoselenides, which commonly arise in oxidations using molar quantities of selenium dioxide, are not encountered. 

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