Halides, purification

The effectiveness of the molten lithium halides purification by hydrogen halides is appreciably lower.This is because molten lithium halides keep water, which is able to dissolve in these melts in considerable quantities. Even mixed halide mixtures retain this property, e.g., the molten KCl-LiCl eutectic strongly keeps the dissolved water at temperatures of the order of 400°C and bubbling of dry HCl during an hour does not result in complete removal of H2O. 

The reaction of monoaIkylarsenic(III) halides and diaIkylarsenic(III) haUdes with ethyleneimine has been described (287). This reaction proceeds successfully, unlike the reaction with AsCl, which can lead to an explosion when purification by distillation is attempted. 

Attempts by Kao and others to enhance transparency by chemically removing impurities from glass met with little success the level of purity required was indeed comparable with that needed in silicon for integrated circuits. In the event, the required purification was achieved in the same way in which semiconductor-grade silicon is now manufactured, by going through the gas phase (silicon tetrachloride), which can be separated from the halides of impurity species because of dilTerences in vapour pressures. This breakthrough was achieved by R.D. Maurer and his... 

Although an old method, the elimination of hydrogen halide from isolated halides is still occasionally recommended. An example is the formation of cholest-2-ene (108) from 3j5-chloro-5a-cholestane, followed by purification via the dibromide (ref. 185, p. 252). 

As a result of their reactivity, particular attention must be given to preparation and purification of the metals, the conditions under which the metals, alloys and compounds are handled and the choice of material for the containment vessel. Ultrapure group-IIB metals may be used without further purification, but it is advisable to purify the group-IIA metals by a multidistillation process, the final distillation preferably being carried out in situ. The reactants and products are best handled in an atmosphere of a purified inert gas, usually He or Ar (N2 cannot be used because of the ready formation of group-IIA metal nitrides) alternatively, they can be handled under vacuum or, in rare cases, under halide fluxes. The containment vessel is normally fabricated from a refractory. 

The platinum metals are valuable by-products from the extraction of common metals such as copper and nickel. The anodic residue that results from copper refining is a particularly important source. The chemistry involved in their purification is too complicated to describe here, except to note that the final reduction step involves reaction of molecular hydrogen with metal halide complexes. 

Rhodium catalyzed carbonylations of olefins and methanol can be operated in the absence of an alkyl iodide or hydrogen iodide if the carbonylation is operated in the presence of iodide-based ionic liquids. In this chapter, we will describe the historical development of these non-alkyl halide containing processes beginning with the carbonylation of ethylene to propionic acid in which the omission of alkyl hahde led to an improvement in the selectivity. We will further describe extension of the nonalkyl halide based carbonylation to the carbonylation of MeOH (producing acetic acid) in both a batch and continuous mode of operation. In the continuous mode, the best ionic liquids for carbonylation of MeOH were based on pyridinium and polyalkylated pyridinium iodide derivatives. Removing the highly toxic alkyl halide represents safer, potentially lower cost, process with less complex product purification. 

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