Halides, metal, also

At 570 K Zn metal is involatile, but at this T it sublimes into a vapor of Zn dichloride. The sublimate analyzes as ca. ZnCI. With similar experiments involving the vapors of ZnBrj and Znl2, Zn metal also sublimes, but analysis of the sublimates shows only ca. 20% of ZnBr from Zn + ZnBrj and 10% of Znl from Zn + Znl2. The monohalides may form at 570 K, but they disproportionate on cooling, in agreement with the known formation of halides ZnX at high T. 

Some of the earliest experimental studies of neutral transition metal atom reactions in the gas phase focused on reactions with oxidants (OX = O2, NO, N2O, SO2, etc.), using beam-gas,52,53 crossed molecular beam,54,55 and flow-tube techniques.56 A few reactions with halides were also studied. Some of these studies were able to obtain product rovibrational state distributions that could be fairly well simulated using various statistical theories,52,54,55 while others focused on the spectroscopy of the MO products.53 Subsequently, rate constants and activation energies for reactions of nearly all the transition metals and all the lanthanides with various oxidant molecules... 

Halides of the less electropositive metals are quickly reduced to highly dispersed and very active metal powders if they are exposed to ultrasonic waves in the presence of lithium and other group I metals(20). Ultrasound not only accelerates the reduction of the halides but also increases the rate of subsequent reactions of these less active metals. These reactions are covered in the chapter by K. Suslick. 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

Silver halide fibres (AgClxBri x) have the widest spectral range in the mid-IR, well into the fingerprint range. Due to their crystalline nature, they have a superior flexibility. Problematic is their tendency to decompose upon contact with UV radiation or base metals. Also sulphides will chemically destroy the fibre material. Other points against are the high intrinsic attenuation due to absorption by impurities or scattering at inclusions or micro-crystals and the non-availability of (applicable) core-clad fibres. 

In the reduction of dienes and polyenes, combinations of a metal hydride and transition metal halides can also be used. Sodium borohydride and cobalt(II) halides were applied in the selective reduction of unsaturated carbon-carbon double bonds93. LiAlH4, in the presence of Zrlv-, TiIV- or Vlv-halides, is a selective reducing agent of dienes94,95. The following reactions were carried out with sodium borohydride and iodine (equation 28)96. 

Different halides were also shown to behave differently under the multiphasic conditions. While bromide was removed faster than chloride, hydrodeiodination became inhibited, likely due to the formation of the ammonium-iodide ion pair (lipophilic and therefore present in the organic phase), which in contact with the Pd/C catalyst poisoned the metal. 

Praesodymium metal can be obtained from its anhydrous halides by reduction with calcium. The metal also may be prepared by electrolysis of fused praesodymium chloride at elevated temperatures (about 1,000°C). Alternatively, an eutectic mixture of praesodymium chloride, potassium chloride, and sodium chloride may be electrolyzed. In such electrolysis graphite is the anode and tungsten the cathode. 

Thorium metal also can he prepared hy thermal reduction of its hahdes with calcium, magnesium, sodium, or potassium at elevated temperatures (950°C), first in an inert atmosphere and then in vacuum. Fluoride and chloride thorium salts are commonly employed. Berzehus first prepared thorium by heating tetrachloride, ThCh, with potassium. Magnesium and calcium are the most common reductant. These metals are added to thorium halides in excess to ensure complete reduction. Excess magnesium or calcium is removed by heating at elevated temperatures in vacuum. One such thermal reduction of hahdes produces thorium sponge, which can be converted into the massive metal by melting in an electron beam or arc furnace. 

Titanium metal also can be produced by electrolytic methods. In electrolysis, fused mixtures of titanium tetrachloride or lower chlorides with alkaline earth metal chlorides are electrolyzed to produce metal. Also, pure titanium can be prepared from electrolysis of titanium dioxide in a fused bath of calcium-, magnesium- or alkali metal fluorides. Other alkali or alkaline metal salts can be substituted for halides in these fused baths. Other titanium com-pouds that have been employed successfully in electrolytic titanium production include sodium fluotitanate and potassium fluotitanate. 

The Heck reactions depicted so far all involve the coupling of halopyridines and other olefins. The alternate approach, coupling of a vinylpyridine with an aryl halide is also feasible, although less commonly employed. 4-Vinylpyridine was coupled successfully with diethyl 4-bromobenzylphosphonate (7.50.) in the presence of a highly active catalyst system consisting of palladium acetate and tn-o-tolylphosphine to give the desired product in 89% yield, which was used for grafting the pyridine moiety onto metal oxides.70... 

The occurrence of transition and rare earth metal hydrides is shown in Figure 1. Not shown are the alkali hydrides and alkaline earth dihydrides that bear a close resemblence to their halide counterparts. Also not shown are compounds formed from elements to the right that are the topic of much of the rest of this symposium volume. 

Aryl- and heteroaryl halides can undergo thermal or transition metal catalyzed substitution reactions with amines. These reactions proceed on insoluble supports under conditions similar to those used in solution. Not only halides, but also thiolates [76], nitro groups [76], sulfinates [77,78], and alcoholates [79] can serve as leaving groups for aromatic nucleophilic substitution. 

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