Halogens reaction with metals

Reactions With Metals. AH metals react to some extent with the halogen fluorides, although several react only superficiaHy to form an adherent fluoride film of low permeabHity that serves as protection against further reaction. This protective capacity is lost at elevated temperatures, however. Hence, each metal has a temperature above which it continues to react. Mild steel reacts rapidly above 250°C. Copper and nickel lose the abHity to resist reaction above 400 and 750°C, respectively. 

Meta.1 Oxides. Halogen-containing elastomers such as polychloropreae and chlorosulfonated polyethylene are cross-linked by their reaction with metal oxides, typically ziac oxide. The metal oxide reacts with halogen groups ia the polymer to produce an active iatermediate which then reacts further to produce carbon—carbon cross-links. Ziac chloride is Hberated as a by-product and it serves as an autocatalyst for this reaction. Magnesium oxide is typically used with ZnCl to control the cure rate and minimize premature cross-linking (scorch). 

Halomet [Halogen metal] A process for reducing halides to metals by reaction with metallic aluminum or magnesium in a closed vessel. Invented in 1968 by R. Nowak and W. Schuster at Halomet, Basle. 

The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Nucleophilic substitution reactions are categorised into and on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of Sj l and Sj 2 reactions. Sj 2 reactions of chiral all l halides are characterised by the inversion of configuration while Sj l reactions are characterised by racemisation. 

The most systematic study of reactions of transition metal atoms with halogen compounds has been the work of Klabunde on oxidation of nickel and palladium atoms. Some work has been done with copper, silver, gold, and platinum, but only scattered results have been reported for other metals. Klabunde s research has shown that perfluoroorgano-halides form isolable organometallic compounds on reaction with metal atoms much more commonly than nonfluorinated halides. The types of reactions observed with different classes of organic halides are considered next. 

Reaction with Metals Halogens react with every metal in the periodic table to yield metal halides. With the alkali and alkaline earth metals, the formula of the halide product is easily predictable. With transition metals, though, more than one product can sometimes form depending on the reaction conditions and the amounts... 

In neither case was it possible to propose definitive mechanisms due to the complexity of the systems in the 7-alumina study, it is suggested that adsorption-desorption processes are slow relative to rapid dismutation between two adsorbed species [105], while from the chromia study mono-molecular halogen exchange reactions with metal halide surface sites are indicated [38], The latter mechanism is reminiscent of the halogen exchange model proposed [95] for C2 CFCs on fluorinated chromia. 

While the Direct Reaction causes the oxidization of a metal and removes it from a solid, it also results in the destruction of carbon-halogen bonds. Since many anthropogenic oi anic chlorides or bromides are major pollutants, reaction with metals provides one potential route for their detoxification. At least one patent has been issued for this application 100). 

Besides substitution by amino, a chloro or bromo substituent may also be replaced by iodo, hydroxy, and alkoxy," though it is not possible to prepare 4-fluoroimidazoles by halogen exchange with metal fluorides on activated bromo- or chloroimidazoles." 4-Fluoroimidazoles are relatively resistant to nucleophiles, but the corresponding 2-fluoro compounds are subject to displacement (via an addition-elimination reaction) under mild conditions (see also Section III,B). Undoubtedly, the intermediate formation of a stable symmetrical 2-fluoroimidazolium cation (120) is responsible for this ease of displacement. (Eq. 28). Iodine at all ring carbons... 

For the synthesis of organyldisilanes, two major synthetic pathways are common (a) reaction of halogenotriorganylsilanes with alkali metals, and (b) reaction of halogen-osilanes with metallated organylsilanes (Scheme 1). The syntheses and applications are treated in another chapter of this volume. 

Certain metal carbonyls are very useful for the removal of halogens from polyhalogenated compounds. In these reactions, again, an oxidative addition step should be important. As shown below, carbenes 33>, cyclobutadiene 34>, and benzyne complexes 35> are formed by the dehalogenation reaction with metal carbonyls. 

Fluorine attacks all organic compounds except CF4. The inorganic compounds that react with fluorine include many, such as asbestos, that are relatively stable to the other halogens. Reaction with water often leads even to explosions. Metals react with fluorine at room temperature some form an impermeable fluorine layer on the surface which protects them against further corrosion, in particular Monel metal, nickel, aluminum, magnesium, iron, and steel. Glass is not attacked by fluorine if it is free from HF and dry. 

Scheme 14 Enantioselective halogenation reactions with different chiral ligand/metal combinations...

The halogens (group 7A) are nonmetals that exist as diatomic molecules. The halogens have the most negative electron affinities of the elements. Thus, their chemistry is dominated by a tendency to form 1— ions, especially in reactions with metals. 

In cyclometalation reactions with metal compounds instead of the metal atoms discussed above, ligands such as hetero atom groups (e.g., bipyridines, phosphines, and carboxylates), unsaturated groups (e.g., aryl, allyl, Cp and Cp ), carbonyl groups, halogen atoms (F, Cl, Br, Cl), etc., also act as metal activators. 

Elements of Group 7A, the halogens, all undergo similar reactions with metals or other nonmetals. 

---