Gray selenium

Removal of the bulky, amorphous selenium is accomplished with the aid of a 6-in. Buchner funnel. The selenium is returned to the reaction flask and extracted with 300 ml. of boiling 95% ethanol for 1 hour (Note 4). The solution, obtained by decantation from the compact gray selenium, is combined with the above filtrate in a 4-1. distilling flask. Distillation under reduced pressure gives two fractions. The lower-boiling fraction (25-60°/16 mm.) consists mainly of ethanol, water, dioxane, and cyclohexanone the higher-boiling one (60-90°/16 mm.) contains cyclohexanone and 1,2-cyclohexanedione with traces of water and dioxane. The yield of crude product is approximately 322 g. 

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. 

Selenium forms several allotropes but tellurium forms only one. The thermodynamically stable form of selenium (a-selenium or gray selenium) and the crystalline form of tellurium are isostructural. In both Te and gray Se, the atoms form infinite, helical chains having three atoms in every turn, the axes of which lie parallel to each other in the crystal, as shown in Fig. 16.8.1. 

Thermolysis of para-substituted diphenylzirconocene 70a in the presence of gray selenium powder gives, in addition to the expected complex 71a, the rearranged product 72a in a ratio of 60 40 55 whereas 70b gives only the rearranged product 72b [Eq. (15)]. Formation of the rearranged product was rationalized by proposing an equilibrium between two isomeric benzyne complexes [Eq. (16)] prior to reaction with selenium. 

These phosphaalkenes 21-24 were reacted with equimolar amount of sulfur or gray selenium to give the related polycycles 25 and 26. In turn, thiaphosphirane 25 was shown to react with another equivalent of sulfur to generate thioxothiaphosphirane 27. 

Tbt- and Bbt-substituted diphosphenes were reacted with gray selenium in a sealed tube in C6D6 at 120°C for 120 h, and this resulted in a mixture of the appropriately substituted orange diphosphaselenirane in 57% yield and... 

As in the procedure in Section A, powdered gray selenium (1.38 g, 15.0 mmol) is treated with Super Hydride (6.0 mL of a 1 M solution of Li[BEt3H] in THF, 6.0 mmol) to give a brown solution of lithium polyselenides. A suspension of TiCp2Cl2 (0.75 g, 3.0 mmol) in dry THF (50 mL) is purged with N2 and added dropwise to the lithium polyselenides. The reaction mixture becomes purple it is stirred under N2 at ambient temperature for 4.75 h. The volatile components are removed in vacuo (0.01 torr). The black residue is extracted under N2 with CH2CI2 (6 x 50 mL), and the solution is filtered under N2 through Celite (5 mL). The purple filtrate is concentrated in vacuo to 50 mL... 

Gray selenium is a photoconductor, which means it conducts electricity more efficiently in the presence of light than in the dark. Some photocopiers use this property to copy images. 

MAJOR APPLICATIONS Gray selenium has an electrical conductivity that increases with temperature and thus exhibits a semiconducting behavior. Selenium has also been demonstrated to be a photoconductor and hence finds applications as selenium photocells used for the measurement of the intensity of light. 

A mixture of 210 g. (95 per cent) potassium cyanide and 240 g. commercial gray selenium is melted in a large... 

FIGURE 50.1 LDI-TOF mass spectra from a deposit of gray selenium recorded under (A) positive-ion-generating and (B) negative-ion-generating modes. 

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