Halogenation of metallated

Anhydrous HX are versatile and vigorous reagents for the halogenation of metals, non-metals, hydrides, oxides and many other classes of compound, though reactions that are thermodynamically permissible do not always occur in the absence of catalysts, thermal initiation or photolytic encouragement, because... 

When the desired halide is hydrolytically unstable then dry methods must be used, often at elevated temperatures. Pre-eminent amongst these methods is the oxidative halogenation of metals (or non-metals) with X2 or HX when more than one oxidation state is available X2 sometimes gives the higher and HX the lower, e.g. ... 

Other routes include the high-temperature halogenation of metal oxides, sometimes in the presence of carbon, to assist removal of oxygen the source of halogen can be X2, a volatile metal halide CX4 or another organic halide. A few examples of the many reactions that have been used industrially or for laboratory scale preparations are ... 

Direct halogenation of metallic bismuth yields bismuth pentafluoride in the case of fluorine, the corresponding bismuth trihalides for the other halogens. Reaction of bismuth trioxide with aqueous HF, HC1, or HBr yields the corresponding bismuth trihalide. Physical and thermochemical properties of the more important bismuth halides appear in Table 2. 

NCS, NBS and NIS are the best reagents for halogenation of metal / -diketonates , the rate of bromination being much faster than that of iodination and chlorination. Two reaction mechanisms have been proposed to explain the halogenation an ionic mechanism, which is operative in polar solvents like chloroform, methanol and acetic acid, and a free radical one occnrring in non-polar solvents like carbon tetrachloride . 

Halogenation of metal isocyanides also can result in metal-metal bond formation e.g., the addition of 1 mol of I2 to 3 mol of [Rh(CNCH2C6H5)4] gives [Rh3(CNCH2C6H5)i2l2]l3 in 85% yield the metal atoms are linear, rather than triangular. ... 

No complete study on the mechanism of halogenation of metal acetyl-acetonates has been carried out. Although the halogenating agents used in these reactions can form the 3-halo compounds via a radical or ionic mechanism, it seems reasonable to assume that these reactions are electrophilic substitution reactions. The probable mechanism is as follows (49, 112) the chelate ring undergoes Sn2 attack by the electrophile ... 

The ligand 8-hydroxyquinoline, which is a phenol, is readily halogenated in the 5- and 7-positions. Studies carried out on the halogenation of metal chelates of 8-hydroxyquinoline and its derivatives show that the effect of the metal ion is generally in accord with what is expected, i.e., the metal ion alters the reactivity of the ligand somewhat but not the orientation of substitution. 

Based on the known heats of formation for NaOH(aq), NaCl(aq), NaBr(aq), Nal(aq), and ZrCUCcr), the author studied the heats of formation of ZrBr4(cr), Zrl4(cr). He used two independent methods the dissolution of the Zr-halides in excess NaOH solutions and dissolution in H2O. Molar ratios of Zr-halide to the aqueous medium were always 1/1500. Corrections of the measured heats of reactions for Hf contents of 1.7% in Zr phases were determined but were found to be within the error of the method. The solid Zr halide samples were prepared by halogenation of metals (X = Br, I) or carbides (X = Cl). The Zr/X ratio in these phases was not analysed for X = (Br, I), but literature data were invoked to show that the ratio is A. For the iodide, the Zr content in the solid was analysed to be within 0.05% of the theoretical value of IZ. 

The halogenation of metals provides opportunities to study scaling reactions in which the reaction product is a pure ionic conductor, which can also be highly volatile. These systems can be studied at relatively low temperature they are important in their own right in terms of the mechanisms of extraction and chemical-vapour-deposition processes that employ gaseous halides as intermediate compounds. 

Hunsdiecker Reactions. The Hunsdiecker reaction is the de-carboxylative halogenation of metal carhoxylate salts. The reaction of a,/3-unsaturated carboxylic acids with NCS and catalytic lithium acetate in acetonitrile-water provides the corresponding /3-halostyrenes in moderate yields under mild conditions. The reaction proceeds with a good degree of stereospecificity. For example, the reaction of 3-(4-methoxy-phenyl)-acrylic acid with NCS with a catalytic amount of lithium acetate at room temperature provides l-(2-chloro-vinyl)-4-methoxybenzene in good yield (eq 23). 3... 

There are numerous alternative routes, including halogen displacement [as in Eq. (7)] and high-temperature halogenation of metal oxides in the presence of carbon or carbon tetrachloride ... 

N-halosucdnimides (chloro-, bromo-, and iodo-) are useful reagents for oxidations in organic chemistry and for the oxidative halogenation of metal complexes. For example, the reaction of the rhenium hydride, trans-(H,Br)-[Re(Bt)(FO(CO)(PMe Cp l with N-hromosucdnimide in THF at 0 C... 

Both halogens of the dihalogenothiazoles can be replaced by nucleophiles. At any rate, the halogen in position 2 is always more reactive than those in positions 4 or 5, as previously discussed. Analogously,the halogen can be selectively removed only from position 2 by reduction with metals (Table V-5). 

Table 8 1 illustrates an application of each of these to a functional group transfer matron The anionic portion of the salt substitutes for the halogen of an alkyl halide The metal cation portion becomes a lithium sodium or potassium halide... 

Alkoxide ion (RO ) The oxygen atom of a metal alkoxide acts as a nucleophile to replace the halogen of an alkyl halide The product is an ether... 

L oss of Catalyst by Vapor Transport. The direct volatilisation of catalytic metals is generally not a factor in catalytic processes, but catalytic metal can be lost through formation of metal carbonyl oxides, sulfides, and hahdes in environments containing CO, NO, O2 and H2S, and halogens (24). 

The quaternary ammonium salts (QAS) are widely used as ionofores of ion-selective electrodes and extractants of metals halogenic anion complexes. The influence of the QASes nature with various methyl groups contents on the cadmium extraction from bromide media has been investigated. 

After the complete removal of halogen and metallic ions, the solution is concentrated to a volume of about 100 cc., and 300 cc. of absolute alcohol is added. Then the amino acid is precipitated by slowly adding 500 cc. of ether with stirring and cooling. 

---