Hydrogen thermal preparation methods

Catalyzed cycloadditions of donor-acceptor-substituted alkenes have been reported in instances in which alkenes are not sufficiently activated for thermal cycloadditions to proceed. The cycloaddition of vinylpyridines to enamincs constitutes a preparative method for the synthesis of aminocyclobutanes.7 This reaction is catalyzed by /Moluenesulfonic acid. The pyridine substituent can be hydrogenated to the piperidine group. 

Uranium hexachloride is a black solid melting at 177.5°. Since it is hygroscopic and reacts vigorously with water, it should be handled only in dry-boxes. The crystal structure has been determined hexagonal symmetry, space group D a-C7> (m, n = 3), with an almost perfect octahedron of chlorine atoms around each uranium atom. Uranium hexachloride can be sublimed at 75-100° at low pressures, but normally some thermal decomposition results. The ultraviolet-visible spectrum of gaseous uranium hexachloride has been determined. No fine structure was observed in the spectrum. Because previously available preparative methods were inadequate, there has been very little study of the chemistry of uranium hexachloride. It reacts with hydrogen... 

The formation of alkali-metal and alkaline-earth-metal sulphides and polysulphides from the elements in liquid ammonia has been extensively studied in the past, but the reactions between the metals and hydrogen sulphide in liquid ammonia have drawn detailed attention only recently. It has been suggested that the equilibrium of H2S in this solvent to give the solvated hydrosulphide ion accounts for the formation of KSH even with an excess of metal. With the alkaline-earth metals, effective preparative methods have been developed for the sulphides from H2S in liquid ammonia but anhydrous hydrosulphides have not been obtained. Now, hydrosulphides have been prepared of the form M(SH)2,xNH3 (M = Ca, Sr, or Ba x — 4, 6, or 0, respectively) from the metals with H2S in ammonia, but the compounds are stable only at low temperatures. Those of Ca and Sr are stable at —45 °C but decompose to the monosulphides at room temperature. Ba(HS)2 decomposes to BaS at 100 °C with evolution of a mole of H2S. For M (SH) (M = Rb or Cs), thermal decomposition gives polysulphides. The hydrosulphides of Rb, Cs, Sr, and Ba hydrolyse rapidly in moist air.74... 

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

Qua.driva.Ient, Zirconium tetrafluoride is prepared by fluorination of zirconium metal, but this is hampered by the low volatility of the tetrafluoride which coats the surface of the metal. An effective method is the halogen exchange between flowing hydrogen fluoride gas and zirconium tetrachloride at 300°C. Large volumes are produced by the addition of concentrated hydrofluoric acid to a concentrated nitric acid solution of zirconium zirconium tetrafluoride monohydrate [14956-11-3] precipitates (69). The recovered crystals ate dried and treated with hydrogen fluoride gas at 450°C in a fluid-bed reactor. The thermal dissociation of fluorozirconates also yields zirconium tetrafluoride. 

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