Selenium compounds oxidation

Preparation and Properties of Selenium(lV) Oxide. Selenium compounds are very poisonous Hand in the prepared substances and residues o/ selenium preparations to the laboratory assistant. Run all experiments with selenium in a fume cupboard After work, thoroughly wash the ware and your hands. 

In many reactions, selenium is an oxidant as well as a reductant. Strong oxidants convert selenium dioxide and its derivatives to the hexavalent state. Although hexavalent selenium compounds are oxidants, these are less active and difficult to reduce. Selenium salts resemble the corresponding sulfur and tellurium salts in behavior. 

Inorganic Compounds. Inorganic selenium compounds are similar to those of sulfur and tellurium. The most important inorganic compounds are the selenides, haUdes, oxides, and oxyacids. Selenium oxidation states are —2, 0, +1, +2, +4, and +6. Detailed descriptions of the compounds, techniques, and methods of preparation, and references to original work are available (1—3,5,6—10, 51—54). Some important physical properties of inorganic selenium compounds are Hsted in Table 3. 

Organic selenium compounds and siUceous materials (rock, ore, concentrates) are fused with mixtures of sodium carbonate and various oxidants, eg, 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 oxidizing fusion usually converts selenium into Se(VI) rather than Se(IV). 

Alkenes can be aminated in the allylic position by treatment with solutions of imido selenium compounds R—N—Se=N—R. The reaction, which is similar to the allylic oxidation of alkenes with Se02 (see 14-4), has been performed with R = t-Bu and R=Ts. The imido sulfur compound TsN=S=NTs has also been used... 

Caution Most selenium compounds are toxic consequently care should be exercised in handling them. The hydrogen peroxide oxidation of selenides is highly exothermic, acid-catalyzed, and auto-catalytic. The procedure given for adding the hydrogen peroxide solution should be carefully followed. 

Stability Unstable in air. Protect from water or moisture. Store away horn heat or ignition sources and sulfur compounds. Reacts with sulfur and sulfur compounds, producing highly toxic VX or VX-like compounds. It completely dissolves polymethylmethacrylate. It is incompatible with calcium hypochlorite (HTH), many chlorinated hydrocarbons, selenium, selenium compounds, moisture, oxidants, and carbon tetrachloride. 

Selenious acid, 24, 61, 62 Selenium, 24, 63 powdered, black, 24, 90 Selenium compounds, ease of oxidation, 24,91... 

Selenurane oxides are also one of the hypervalent selenium compounds. Recently, the enantiomers of chiral selenurane oxide 38 were isolated for the first time by enantioselective liquid chromatography of the racemate or by spontaneous resolution occurring during slow evaporation of its acetonitrile solution or slow crystallization from the same solvent.57 The absolute configurations of the enantiomers were determined by X-ray crystallographic analysis (Scheme 17). 

Urea complex with hydrogen peroxide (UHP) seems to be a very useful and convenient oxidation system in nonaqueous medium. The UHP reaction proceeds in methanol and is catalyzed by Mo (VI), W (VI) salts, or SeC>2 (83). Catalytic oxidation in a water-free medium can be carried out with alkylhydroxyperoxides, the catalysts being titanium alkoxides (84) or selenium compounds (85). The reaction appears to proceed quickly and with good selectivity. 

Not only propargyl precursors but also acceptor-substituted 1-methyleneallyl compounds such as 67, 71 or 74 can be used to produce the target allenes by sigmatropic reactions (Scheme 7.10). After oxidation of selenium compounds 67 followed by equilibration of the resulting selenoxides 68 and selenenic esters 69, hydrolysis... 

The chemical properties of selenium fall between sulfur and tellurium. Thus, selenium reacts with oxygen similarly to sulfur, forming two oxides, selenium dioxide, Se02 and trioxide, SeOs. The metal combines with halogens forming their halides. With nonmetals, selenium forms binary compounds exhibiting oxidation states +4 and -i-6. 

Selenium dioxide is used to make other selenium compounds and as an oxidizing agent. 

Inorganic selenium compound Se 3d3/2,5/2 eV q Oxidation (effective charge) number ... 

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