Preparation of Fluorides

The reagents useful for the fluorination of ores and compounds include flurosilicates, elemental fluorine, hydrogen fluoride, and alkali hydrogen fluorides. Their use is illustrated for fluorination of oxides and oxidic minerals. 

The fluorination of metal oxides by fluorine, represented by the reaction 

One of the most important examples of the fluorination of oxides is the fluorination of uranium dioxide. Uranium tetrafluoride (UF4) is the intermediate compound which is reduced to uranium metal. The gaseous higher fluoride, uranium hexafluoride (UF6) is used for the separation of uranium isotopes to obtain enriched uranium (i.e., uranium containing a higher proportion of the isotope, U235, than natural uranium). 

The following reactions can be considered for the preparation of uranium fluorides (UF4 or UF6) starting from urania (U02) at a temperature of 500 °C  

Plutonium tetrafluoride is prepared from plutonium dioxide by the following reaction at temperatures between 400 and 600 °C  

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Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

Treatment of diazonium salts with cuprous chloride or bromide leads to aryl chlorides or bromides, respectively. In either case the reaction is called the Sandmeyer reaction The reaction can also be carried out with copper and HBr or HCl, in which case it is called the Gatterman reaction (not to be confused with 11-16). The Sandmeyer reaction is not useful for the preparation of fluorides or iodides, but for bromides and chlorides it is of wide scope and is probably the best way of introducing bromine or chlorine into an aromatic ring. The yields are usually high. 

For the preparation of fluorides and iodides from diazonium salts, see 13-23 and 13-22. 

In fluoride glasses, calcium fluoride is an essential constituent, but generally cryolite, NajAlFg, is also added as a flux to lower the temperature of fusion. Aluminium orthophosphate is also generally added to the fusion mixture for various reasons. Of course, the various elements may be added in different ways. Thus, calcium orthophosphate, aluminium fluoride and sodium carbonate are often used in the preparation of fluoride glasses. 

A comparison of various fluorinating agents in the preparation of fluoride 13, i.e. (2-chloro-1,1,2-trifluoroethyl)diethylamine and diethylaminosulfur trifluoride (R = H), tetrafluoro(phen-yl)-A5-phosphane (R = TMS), and potassium fluoride (R = Ts), has shown that the highest stereoselectivity is achieved with potassium fluoride and (2-chloro-l,1,2-trifluoroethyl)-diethylamine.19 The fluorination with tctrafluoro(phenyl)-7.5-phosphane proceeds, at least in part, via a carbocation and with a relatively low enantioselectivity.15... 

Preparation of fluorides aluminum trifluoride, uranium tetra- and hexafluorides... 

The fluoride ion complex of 5-octamethyl-[12]-mercuracarborand-4 26 is an effective source of naked fluoride <2004AGE1854>. Treatment of a tosylated sugar with fluoride 26, in the presence of tetra- -butyl ammonium iodide 27, leads to 49% tosylate replacement. The reaction does not occur without co-reagent 27, so presumably its role is to help release fluoride ion from complex 24. Scheme 3 illustrates the preparation of fluoride reagent 26 by decomplexa-tion of diiodide 25 with AgBp4 forming neutral tetramer 24, followed by treatment with tetramethylammonium fluoride. 

Various techniques may be employed for the preparation of fluoride fibers and preforms [152]. The basic steps involved in the fabrication of preforms suitable for singlemode fiber are summarized in Fig. 13. 

Calcination of aluminum hydroxide to alumina Preparation of aluminum sulfate from bauxite Preparation of fluorides aluminum trifluoride, uranium tetra- and hexafluorides... 

Bromine pentafluoride is the most reactive of the AXg compounds, resembling CIF3 in acting very violently, too violently to be used, undiluted, for the preparation of fluorides. Liquid IF5 is a good conductor ... 

Common Operational Problems in Preparation of Fluoride Solution... 

There are also potentially important inorganic applications of COF. Most of the industrially important low-oxidation-state metal fluorides are prepared from the reaction of their oxides, hydroxides or carbonates with aqueous or anhydrous HF. However, preparation of fluorides of metals in high formal oxidation states is usually achieved by the use of elemental fluorine on the metal, because of the problem of hydrolysis from the reversible reaction of the oxide with HF ... 

The oxalate precipitation and subsequent calcination to the oxide is reserved for multimicrogram and greater quantities of actinides and for Es-253, whose intense radiation precludes the use of the resin bead technique. The preparation of fluoride compounds is not carried out in quartz but in Monel (6) Details of the conditions of temperature, pressure, etc. to effect these chemical reactions are available in the literature Cl>3,6-11) ... 

Titanium tetrachloride is an efficient reagent for the conversion of trialkyl phosphites and dialkyl hydrogen phosphonates into dialkyl phosphorochloridates. Imidazolides and dialkyl or diaryl phosphoric acids react with acyl fluorides - benzoyl fluoride and oxalyl difluoride being the reagents of choice - to give quantitative yields of the phosphoryl fluorides. The procedure is adaptable to the preparation of fluorides of carbohydrate phosphates in this field, the reaction between the ceu bohydrate and tris-l//-imidazolylphosphine oxide or sulphide with the replacement of one imidazole... 

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