Hydrogen halides, purification

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). 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

Lagowski, J. J., Ed., The Chemistry of Non-Aqueous Solvents, Academic, New York. This series contains detailed accounts of the purification, properties, and handling of some major solvents Vol. 2(1967), hydrogen halides, amides, and ammonia Vol. 3(1970), sulfur dioxide and acetic acid Vol. 4 (1976), tetramethylurea, cyclic carbonates, and sulfolane Vol. 5A (1978), tri-fluoroacetic acid, hafosuffuric acids, interhalogens, inorganic halides and oxyhalides. 

The effectiveness of the purification of molten lithium halides by hydrogen halides is appreciably lower than mentioned above [232], This is explained by the fact that molten lithium halides keep the water, which can dissolve in these melts in considerable quantities. Even mixed-halide mixtures retain this property, e.g. the molten KCl-LiCl eutectic keeps the dissolved water strongly at temperatures of the order of 400 °C [179], and bubbling of dry HC1 during an hour does not result in complete removal of H20. 

The introduction of double bonds into organic systems via the elimination of hydrogen halides is a widely applicable transformation [137] (see also Sect. 3.4.2.1). Dehalogenations have been used as a means for the purification of olefins, for the temporary protection of double bonds, and for generating a new double bond as part of a synthetic sequence[137]. 

Pressure gasification A mixture of CO, CO2, and hydrogen halides is formed at ca. 1600 °C. The hydrogen halides must be separated by waste-gas purification to recover the halogens. The formation of elementary bromine in the gasification process is problematic and would lead to considerable difficulties in waste-gas purification. 

The effectiveness of the purification of melts based on lithium salts by hydrogen halides is much lower. Water dissolves in the KCl-LiCl melt in appreciable amounts and is firmly retained at temperatures up to 400 C. When dry HCl is passed for 1 h, removal of H2O is incomplete. 

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. 

All the above-mentioned initiators are very sensitive towards substances with active hydrogen. Care must therefore be taken to exclude acids, water, thiols, amines, and acetylene derivatives. Oxygen, carbon dioxide, carbon monoxide, carbonyl compounds, and alkyl halides which can react with the initiator, also interfere with the reaction. Careful purification and drying of the starting materials and apparatus is, therefore, absolutely essential, especially when dealing with living polymers (see Example 3-19). 

Formation and Reduction of Azides Azide ion ( N3) is an excellent nucleophile that displaces leaving groups from unhindered primary and secondary alkyl halides and tosylates. The products are alkyl azides (RN3), which have no tendency to react further. Azides are easily reduced to primary amines, either by LiAlH4 or by catalytic hydrogenation. Alkyl azides can be explosive, so they are reduced without purification. 

Alkenylboron compounds couple with the representative organic halides or triflates (Scheme 28). Hexaalkylbenzene was synthesized by sixfold alkenylation (55) of hexabromobenzene followed by catalytic hydrogenation of the double bonds 11521. The reaction of 1-alkenylborane with 1-bromo-l-alkyne stereose-lectively provided ( )-enyne (56) which was then converted into ( ,Z)-hexa-deca-10,12-dienal, a sex pheromone of the melonworm 11531. Due to the difficulty of purification of a geometrical mixture, the stereoselective synthesis is critical for such dienes or trienes. The PGEi derivatives (57) were synthesized... 

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