1,1.1-Trihalides special

The halogenolysis of diorganoditellurides is very fast and the organotellurium trihalides precipitate immediately, usually as light yellow solids in the case of the chlorides, as yellow solids in the case of the bromides, and as red-brown solids in the case of iodides. Some trihalides, specially the aliphatic derivatives, precipitate as dense oils, which crystallize by addition of appropriate solvents. 

The importance of the trihalides as industrial chemicals stems partly from their use in preparing crystalline boron (p. 141) but mainly from their ability to catalyse a wide variety of organic reactions.BF3 is the most widely used but BCI3 is employed in special cases. Thus, BF3 is manufactured on the multikilotonne scale whereas the production of BCI3 (USA, 1990) was 250 tonnes and BBr3 was about 23 tonnes. BF3 is shipped in steel cylinders containing 2.7 or 28 kg at a pressure of 10-12 atm, or in tube trailers... 

Binary rare-earth compounds such as carbides, sulfides, nitrides, and hydrides have been used to prepare anhydrous trihalides, but they offer no special advantage. Treating these compounds at a high temperature with a halogen (98) or hydrogen halide (115) produces the trihalide, e.g.,... 

Oxyfluorides are the most stable of the oxyhalides, and the stability decreases with increasing atomic weight of the halogen. Special methods are used in the preparation of oxyfluorides, while the other oxyhalides can be prepared by general reactions, such as the partial hydrolysis of the pentahalides, or the oxidation of the trihalides. The compounds fume in the air and readily undergo further hydrolysis giving hydrogen halides and oxyaeids of phosphorus. 

The major types are the trihalides (EX3), the pentahalides (EX5), and the dielement tetrahalides (E2X4). We shall discuss them in that order. All trihalides except PF3 are best obtained by direct halogenation, keeping the element in excess, whereas the pentahalides may all be prepared by reaction of the elements with excess halogen. Special methods are used for E2X4 compounds. 

The large number of recent publications on preparative methods suggests that this is an important aspect of halide chemistry. High purity materials with low levels of anionic contamination are naturally desired for property measurements. The trihalides occupy a position of special importance because they are employed as starting materials for the preparation of essentially all the tetrahalides and reduced halides. High quality anhydrous halides are commercially available, but care must be exercised in their procurement (Haschke, 1975a). The reactivity of the chlorides, bromides and iodides to atmospheric moisture virtually necessitates that they be prepared in the laboratory. Preparative procedures have been described in several comprehensive reviews (Taylor, 1962 Kiss, 1963 Brown, 1968). Additional reviews have appeared for the fluorides (Carlson and Schmidt, 1961 B tsanova, 1971) and for the chlorides, bromides and iodides (Block and Campbell, 1961 Johnson and Mackenzie, 1970). In the present review, an attempt will be made to evaluate the various methods. 

The important preparative methods for the trihalides may be included in one of the following categories (a) preparation from the rare earth metals, (b) preparation from other halides by metathetical processes, and (c) preparation from the oxides. Since the hydrated halides are readily prepared from the oxides, their dehydration is considered as a special case of category (c). A variety of additional preparative methods appears in the literature, and many of these are described in the reviews mentioned above. The starting materials are frequently so unusual and the procedures so complex that they are of little synthetic value. 

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