Oxygen elemental selenium

FIGURE 1.62 The Croup 16A/I elements. From left to right oxygen, sulfur, selenium, and tellurium. Note the trend from nonmetal to metalloid. 

The chalcogenides are binary compounds of a chalcogen (i.e., the elements of Group Ilb zinc, cadmium, mercury) with a less electropositive element, such as those of Group VIb (oxygen, sulfur, selenium, and tellurium). This section covers the sulfides, selenides, andtellurides. Oxides are reviewed above in Ch. 11. Most of the chalcogenides have useful optical characteristics and their applications are usually found in optics. 

Berzelius determined the atomic weights of nearly all the elements then known, and was the first chemist to determine them accurately. (19). He referred his atomic weights to oxygen, which, however, he allowed to equal 100, instead of 16 as in our present system. In his little laboratory that looked like a kitchen and in which the sandbath on the stove was never allowed to cool, Berzelius discovered the important elements selenium, silicon, thorium, cerium, and zirconium (18). 

Selenium is an essential trace element in the human body. This nutrient is an important part of antioxidant enzymes that protect cells against the effects of free radicals that are produced during normal oxygen metabolism. Selenium is also essential for normal functioning of immune system and thyroid gland. 

The oxidation of 2-butene with selenium dioxide in acetic acid solution produces l-acetoxy-2-butene as the oxidation product of the olefin and bis(l-methyl-2-acetoxypropyl)-selenide (I) as the final reduced state of the oxidant instead of elemental selenium. Further, (I) may act as a catalyst for the oxidation of 2-butene with peracetic acid or oxygen to 3-acetoxy-1-butene. A mechanism is proposed to explain the formation of selenides in the oxidation of olefins with selenium dioxide and their catalytic activity in the oxidation of olefins with other oxidants. 

Conductivity measurements in N/10 aqueous solution show the dissolved gas to be ionised to the extent of 50 per cent., whilst hydrogen selenide in Nj 10 solution is only 4-1 per cent, ionised. The acidity of the hydrides of the elements oxygen, sulphur, selenium and tellurium therefore falls into the regular series II2Te >H2Se >H2S >II20, in inverse order to the stability.6... 

The group 6A elements are oxygen, sulfur, selenium, tellurium, and polonium. As shown in Table 19.7, their properties exhibit the usual periodic trends. Both oxygen and sulfur are typical nonmetals. Selenium and tellurium are primarily non-metallic in character, though the most stable allotrope of selenium, gray selenium, is a lustrous semiconducting solid. Tellurium is also a semiconductor and is usually classified as a semimetal. Polonium, a radioactive element that occurs in trace amounts in uranium ores, is a silvery white metal. 

Elements of Group VIA (Oxygen, Sulfur, Selenium, and Tellurium)... 

Selenium dioxide is able to a-oxygenate ketones via their enol tautomers. As is demonstrated in Figure 12.10 by the reaction of selenium dioxide with cyclohexanone, the actual electrophilic substitution product C is unstable. The latter contains selenium in the oxidation state +2 that takes the opportunity to transform into selenium in the oxidation state 0, i.e., elemental selenium, by way of the fragmentation reaction indicated. Thereby, the a-C O single bond of the primary product C is transformed into the a-C=0 double bond of the final product B (which, however, is largely present as the tautomeric enol A). 

C. Rodger, N. Sheppard, H. C. E. McFarlane and W. McFarlane, Group VI — Oxygen, sulfur, selenium and tellurium, in NMR and the Periodic Table, R. K. Harris and B. E. Mann, eds., Academic Press, London, 1978. O. Lutz, Group VI elements other than oxygen, in The Multinuclear Approach to NMR Spectroscopy, J. B. Lambert and F. G. Riddell, eds., NATO ASI Series, D. Reidel Publishing Company, Dordrecht, 1982. H. C. E. McFarlane and W. McFarlane, Sulfur, selenium, and tellurium, in Multinuclear NMR, J. Mason, ed., Plenum Press, New York, 1987. 

Few examples of photooxidation of selenides are known. Hiotolysis of dilute aerated solutions of dibenzyl diselenide in benzene resulted in the formation of benzaldehyde and elemental selenium. i Without oxygen present during the photolysis only decomposition to dibenzyl selenide and selenium was observed. i 1 Photolysis in CDCb in an NMR tube in the presence of oxygen gave complex mixtures of products derived fnm benzyl radicals. It appears that monoselenides are more stable dian die corresponding sulfides and teUurides to phouxixidadon. ... 

In alkaline medium it is hydrolyzed to ammonia, elemental selenium, and selenium-oxygen compounds. 

There have been two published reports on the syntheses of stable 1,2,4-triazolyl carbenes. Thermal decomposition in vacuo of 5-methoxytriazoline 208 provided in quantitative yield l,2,4-triazol-5-ylidene 209, a stable carbene in the absence of oxygen and moisture <0381292>. This nucleophilic carbene 209 could react with a variety of alcohols, thiols, amines, oxygen, sulfur, selenium, isocyanantes, and metal carbonyls to form a myriad of addition products. Reactions of 1,2,4-triazolyl perchlorate salts 210 with base afforded stable nucleophilic 1,2,4-triazol-5-ylidenes 211, which could react with acetonitrile and elemental sulfur and selenium to yield addition products <03JOC5762>. 

Other chemists then showed that the elements could be classified into groups coi sisiing of hiore than three similar elements. Fluorine was added to the tri ad of "chlorine, bromine, and iodine, and magnesium to the triad calcium, strontium, barium. Oxygen, sulfur, selenium, and tellurium had been classed as one family, and nitrogen, phosphorus, arsenic, antimony, and bismuth as another family ot elements by 1854. 

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