Thermal halides

Fatemi F K, Fatemi D J and Bloomfield L A 1996 Thermal isomerization in isolated oesium-halide olusters Phys. Rev. Lett. 77 4895... 

These values indicate a rapid fall in thermal stability of the halide from fluorine to iodine, and hydrogen iodide is an endothermic compound. If we now examine the various enthalpy changes involved. we find the following values (in kJ) ... 

Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament. 

PVF is more thermally stable than other vinyl halide polymers. High molecular weight PVF is reported to degrade in an inert atmosphere, with concurrent HF loss and backbone cleavage occurring at about 450°C (71,72). In air, HF loss occurs at about 350°C, followed by backbone cleavage around 450°C. 

Hafnium Halides. Hafnium tetrafluoride, HfF, can be prepared by careful thermal decomposition of ammonium fluorohafnate in an... 

Carboxylic acid hydiazides are prepared from aqueous hydrazine and tfie carboxylic acid, ester, amide, anhydride, or halide. The reaction usually goes poody with the free acid. Esters are generally satisfactory. Acyl halides are particularly reactive, even at room temperature, and form the diacyl derivatives (22), which easily undergo thermal dehydration to 1,3,4-oxadiazoles (23). Diesters give dihydtazides (24) and polyesters such as polyacrylates yield a polyhydrazide (25). The chemistry of carboxyhc hydrazides has been reviewed (83,84). 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

Figure 22 represents the main consequences on pyrazole reactivity when a positive charge is present at the 2-position of the nucleus. A similar situation occurs in the indazolium salts, which thermally decompose into an alkylindazole and an alkyl halide, a reaction sequence described by von Auwers. 

Material Moist, e.g.. chlorine below dew point F)ry, e.g., fluorine above dew point Hydrogen halides, dry,J e.g., dry hydrogen cliloride, F Available forms Cold formability in wronglit and clad form Weldability Maximum strength annejiled condition x 1000 Ib/in- Coefficient of thermal expansion, millionths per F, 70-212 F Remarks ... 

An extension ot this reaction provides a number of other perfluorovinylic halides [54] The type of reaction products from the thermal decomposition reaction and the type of hydrocarbon Grignard reagent used in the exchange reaction are solvent-dependent When an excess ot phenylmagnesium bromide is used, a variety of phenylated products are formed depending on the excess amount used [4S (equation 23)... 

An efficient catalyst for thermal isomenzations of halofluorocarbons [6, 7, 8, 9] IS prepared by treatment of alumina with dichlorodifluoromethane at 200-300 °C [9] or aluminum chloride with chlorofluorocarbons in the presence of metals [W] or palladium on alumina [II These catalysts are far more efficient than aluminum halides themselves (equations 1 and 2)... 

The reactions of trisilylated benzamidines with benzenechalcogenyl halides are of particular interest. The initially formed trisubstituted derivatives are thermally unstable and decompose by a radical process to generate the dark blue diazenes tran5-PhEN(Ph)CN=NC(Ph)NEPh (E = S, Se) (Scheme 2.3) (Section 15.2)." ... 

Monomeric thiazyl halides NSX (X = F, Cl Br) have been characterized in the gas phase, but oligomerization to cyclic species, e.g., (NSX)3 (X = F, Cl) and (NSF)4, occurs in the condensed phase (Section 8.7). These ligands can be stabilized, however, by coordination to a transition metal. The NSF complexes are conveniently prepared in SO2 (Eq. 1.6) The monomeric fluoride NSF is conveniently generated in situ by thermal decomposition of FC(0)NSF2 or Hg(NSp2)2 (Section 8.2). 

Anhydrous beryllium halides cannot be obtained from reactions in aqueous solutions because of the formation of hydrates such as [Be(H20)4]F2 and the subsequent hydrolysis which attends attempted dehydration. Thermal decomposition of (NH4)2Bep4 is the best route for BeFr, and BeCl2 is conveniently made from the oxide... 

Alternatively, thermal decomposition or disproportionation can yield the lower halide ... 

By contrast, ZrCl and ZrBr, also prepared by the high temperature reduction of ZrX4 with the metal, appear to be genuine binaiy halides. They are comprised of hep double layers of metal atoms surrounded by layers of halide ions, leading to metallic conduction in the plane of the layers, and they are thermally more stable than the less reduced phases. Zrl has not been obtained, possibly because of the large size of the iodide ion, and, less surprisingly, attempts to prepare reduced fluorides have been unsuccessful. 

The highest halide of each metal is of course a fluoride Rep7 (the only thermally stable heptahalide of a transition metal), TcFg, and MnF4. This again indicates the diminished ability of manganese to attain high oxidation states when compared not only to Tc and Re but also to... 

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