Group 16 elements selenium

Although the sulfur-gold bond has been most investigated, the Group 16 elements selenium and tellurium have also attracted attention and are discussed in detail here (polonium has not received attention due to its radioactivity). 

FIGURE 15.14 Two of the Group 16 elements selenium on the left and tellurium on the right. 

Vol 12 PerfluorohaloorganiL Compounds of Main Group Elements Part 2 Compounds of Sulfur (Cuniinuanon) Selenium and Tellurium... 

In each representation, the alkali metal is black, the rare-earth element is shaded medium grey, the main group element is dark grey, and selenium is light grey (a) NagEu2(Si2Se6)2,... 

The oxidative carbonylation of amines to give ureas is at present one of the most attractive ways for synthesizing this very important class of carbonyl compounds via a phosgene-free approach. Ureas find extensive application as agrochemicals, dyes, antioxidants, resin precursors, synthetic intermediates (also for the production of carbamates and isocyanates), and HIV-inhibitors. Many transition metals (incuding Au [244], Co [248,253-255], Cu [242], Mn [249,256-258], Ni [259], Rh [246,247,260-262], Ru [224,260,263] and especially Pd [219,225,226,264-276], and, more recently, W [277-283]) as well as main-group elements (such as sulfur [284-286] and selenium [287— 292]) have been reported to promote the oxidative carbonylation of amines, usually under catalytic conditions. In some cases, carbamates and/or oxamides are formed as byproducts, thus lowering the selectivity of the process. 

Wachsmann M, Heumann KG (1992) Negative thermal ionization mass spectrometry of main group elements, part 2. 6th group Sulfur, selenium, and tellurium. Int J Mass Spectrom Ion Proc 114 209-220... 

The thionucleoside with a free 5 -hydroxy group was phosphitylated in the presence of tetrazole or TMCS (step a) and the intermediate formed coupled with 5 -0-acetylthymidine activated by DBU (step b). The addition of elemental selenium (step c) is followed by the fast demethylation in the presence of 4-nitrophenoxide (step d). All these operations can be performed as a one-flask procedure to give the desired product in over 90% yield. 

Among the oxygen group elements, while sulfur is oxidized to +6 oxidation state (in H2SO4), selenium and tellurium are oxidized to +4 oxyacids with the liberation of nitrogen dioxide ... 

The next five chapters deal with deposition of specific groups of semiconductors. In Chapter 4, II-VI Semiconductors, all the sulphides, selenides, and (what little there is on) tellurides of cadmium (most of the chapter), zinc (a substantial part), and mercury (a small part). (Oxides are left to a later chapter.) This chapter is, understandably, a large one, due mainly to the large amount of work carried out on CdS and to a lesser extent on CdSe. Chapter 5, PbS and PbSe, provides a separate forum for PbS and PbSe, which provided much of the focus for CD in earlier years. The remaining sulphides and selenides are covered in Chapter 6, Other Sulphides and Selenides. There are many of these compounds, thus, this is a correspondingly large chapter. Chapter 7, Oxides and Other Semiconductors, is devoted mainly to oxides and some hydroxides, as well as to miscellaneous semiconductors that have only been scantily studied (elemental selenium and silver halides). These previous chapters have been limited to binary semiconductors, made up of two elements (with the exception of elemental Se). Chapter 8, Ternary Semiconductors, extends this list to semiconductors composed of three elements, whether two different metals (most of the studies) or two different chalcogens. 

Oxidation of 2-Butene with Selenium Dioxide. The stoichiometry of the reaction of 2-butene with selenium dioxide shows that approximately 0.85 mole of l-acetoxy-2-butene plus 0.85 mole of bis(l-methyl-2-acetoxypropyl) selenide are produced per mole of selenium dioxide consumed. This suggests that, at least for this particular group of olefins, in the mechanism of olefin oxidation with selenium dioxide the formation of selenides should be considered as the final reduced state of the oxidant rather than elemental selenium. 

The pattern of ion formation by main-group elements can be summarized by a single rule for atoms toward the left or right of the periodic table, atoms lose or gain electrons until they have the same number of electrons as the nearest noble gas atom. Thus, magnesium loses two electrons and becomes Mg2+, which has the same number of electrons as an atom of neon. Selenium gains two electrons and becomes Se2, which has the same number of electrons as krypton. We shall discover the origin of this rule in Section 2.3. 

As a member of group 6 of the periodic table of elements, selenium shares many chemical properties with its neighboring homolog, sulfur see Selenium Inorganic Chemistry, Selenium Organoselenium Chemistry). Both... 

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