Halogens, reduction potentials

Amine—borane adducts have the general formula R3N BX where R = H, alkyl, etc, and X = alkyl, H, halogen, etc. These compounds, characterized by a coordinate covalent bond between boron and nitrogen, form a class of reducing agents having a broad spectmm of reduction potentials (5). 

Although it is only slowly oxidized in moist air at ambient temperature, cadmium forms a fume of brown-colored cadmium oxide [1306-19-0] CdO, when heated in air. Other elements which react readily with cadmium metal upon heating include the halogens, phosphoms, selenium, sulfur, and tellurium. The standard reduction potential for the reaction... 

From measurements of this type Thomaz and Stevens found a linear relationship for a graph of log(kJrPC,2) vs. where n is the number of halogen atoms in the molecule, is the spin-orbit coupling constant, and Em is the polarographic half-wave reduction potential of the heavy-atom quencher (Figure 5.16). This correlation suggests that an exciplex is formed by partial... 

F2 has the most positive standard reduction potential and therefore is the strongest of all common oxidizing agents. Oxidizing strengths of the diatomic halogen molecules decrease down Group 7A. 

The standard reduction potential for Be2+ is the least negative of the elements in the group and by the same token beryllium is the least electropositive and has the greatest tendency to form covalent bonds. The bulk metal is relatively inert at room temperature and is not attacked by air or water at high temperatures. Beryllium powder is somewhat more reactive. The metal is passivated by cold concentrated nitric acid but dissolves in both dilute acid and alkaline solutions with the evolution of dihydrogen. The metal reacts with halogens at 600°C to form the corresponding dihalides. 

Tlte reduction potential for an alkyl or benzyl radical, relative to that of the carbon-halogen bond from which it is derived, is important in determining the isolated products. Products are derived either by radical or by carbanion chemistry. The half-wave potential for the second polarographic wave of alkyl halides is connected with reduction of the radical. Sophisticated methods have been devised for deducing radical reduction potentials in cases where (his second wave is not seen. Values are collected in Table 4.4. 

The standard reduction potentials for the main species formed by the Group 17 elements in aqueous solution are given in Tables 6.16 and 6.17, for pH values 0 and 14, respectively. Irrespective of the pH of the solution, the halogen elements range from the extremely powerful F2 (which has the potential to oxidize water to dioxygen), through the powerful oxidants Cl2 and Br2, to 12, which is a relatively weak oxidant. 

Figure 14.4 Selection of environmentally relevant redox couples including organic pollutants such as nitroaromatic and halogenated compounds, as well as examples of electron transfer mediators and important bulk reductants. The values given represent reduction potentials at pH 7 at equal (except otherwise indicated) concentrations of the redox partners but at environmental con-centrations of the major anions involved ...

C. E. Ophardt, "Redox Demon-f strations and Descriptive Chemistry Part 2. Halogens/ /. Chem. Educ., Vol. 64,1987,807. Using an abbreviated table of reduction potentials as a predictive tool, reactions of bromine and iodine in various oxidation states are demonstrated. 

The reduction potentials for various alkyl halides range from +0.5 to +1.5 V therefore, when Fe° serves as an electron donor, the reaction is thermodynamically favorable. Because three reductants are present in the treatment system (Fe°, H2, and Fe2+), three possible pathways exist. Equation (13.9) represents the oxidation of Fe° by reduction of a halogenated compound. In the second pathway, the ferrous iron behaves as a reductant, as represented in Equation (13.10). This reaction is relatively slow because the ability to reduce a pollutant by ferrous iron is dependent on the speciation ferrous ions, which is determined by the ligands present in the system. The third possible pathway, Equation (13.11), is dehalogenation by hydrogen. This reaction does not occur easily without a catalyst. In addition, if hydrogen levels become too high, corrosion is inhibited (Matheson and Tratnyek, 1994) ... 

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