Electron transfer 37 Haber

Following the discovery of Haber and Weiss [78] that Fe(II) salts re ct with H2O2 by one electron transfer process to give OH, Baxandale et al. [79] used the Fe - H2O2 system for effecting polymerization of vinyl monomers. 

Haber and Weiss postulated an electron transfer mechanism for the ferrous ion catalyzed decomposition of peroxides, the metal donating an electron... 

Now let us estimate the low boundary, Wiow, of the Ore gap in molecular liquids. Because the Ore process is just an electron-transfer reaction, we assume that no rearrangement of molecules occurs and, therefore, the final positronium state will be quasi-free (formation of the bubble requires much longer time). The corresponding Born-Haber cycle is the following ... 

The experimental (Born-Haber) and theoretical (Born-Mayer) estimates of lattice energy agree well for typically ionic compounds such as the Gp.IA halides, and show that the theoretical picture of complete electron transfer is satisfactory. Values for salts of 18-electron type cations, however, often show considerable discrepancies. Representative differences between C/ xpi and are Rbl, 4 kcal Cdig, 86 kcal PbOg, 212 kcal. These indicate a gradual departure from the purely ionic condition the small cations tend to retain some hold on their electrons and the binding acquires considerable covalent character. 

The overall reaction is initiated by an electron transfer from ascorbate to 4-NQO with the production of A and 4-NQO" (Reaction 46, l). The 4-NQO" radical reacts rapidly [reported values for similar compounds range from 10 to 10 M V (62,63)] with molecular oxygen to yield superoxide radical (Reaction 46, 2), which dismutates to peroxide and oxygen (Reaction 46, 3) or reacts with ascorbate (Reaction 46, 4). Specific tests with superoxide dismutase and catalase suggest that OH radicals are formed in this system by a Haber-Weiss (64) and/or Fenton (65) type reaction ... 

From a thermodynamic point of view, the variation of standard free enthalpy associated to the electron transfer process represented by Equation (2.1), tAG °, can be related with the variation of such tliermodynamic quantity for the electron transfer process for species in solution phase, lAG °. and for the transfer of the oxidized, IaG, and reduced, forms of the electroactive species and the electrolyte cations, from the solution phase to the porous solid. The corresponding Bom-Haber-type cycle is shown in... 

The reduced species Fe2+ and Mn2+ have been detected electrochemically in anoxic waters (De Vitre et al., 1988). Vanadyl (V02 + ) is known to be incorporated in geoporphyrins in organic-rich sediments (Eckstrom et al., 1983). Moffett and Zika (1988) measured reduced Cu(I) photometrically in surface waters of the open ocean. The oxidation of these four metal species involves a simple one-electron transfer step. Haber and Weiss (1934) proposed a kinetic mechanism for the oxygenation of the ferrous ion, in which the first step in the four-electron reduction of the dioxygen molecule determines the rate. The redox potentials for the corresponding oxygen couples support this view they are plotted ill... 

The results obtained with different mixing devices led Haber and Weiss (3) to the formulation of the reaction as a radical and chain reaction. If Fen-salt is at all times in excess the mechanism can be represented by the two simple electron transfer processes ... 

Figure 2. Born-Haber cycle for the single-electron transfer (SET) response of the carbon-metal bond, incorporating the Blicke-Powers hypothesis.

Oxidation of Organic Contaminants with Hydrogen Peroxide Catalyzed hy AC Recently, there has been renewed interest in the catalytic system H2O2/AC, due to its potential application in the oxidation of organic contaminants in water and wastewater. The key factor for this application is the formation of free radicals [see reactions (6.12) and (6.13)] which are known to be very strong oxidants in the liquid phase. Carbon materials can act as electron-transfer catalysts, similar to the Haber-Weiss mechanism known from the Fenton reaction, with AC and AC" as the reduced and oxidized catalyst states [161] ... 

As early as 1932, Haber and Weiss [11] suggested a mechanism of interaction between hydrogen peroxide and iron(II) ions where electron transfer to the peroxide molecule led to the formation of a free hydroxyl radical ... 

Any cell reaction can be considered to be an electron transfer between two coupled half-cells. The measured potential corresponds to the difference of the electron energy. The arbitrary definition of a reference electrode raises the question of whether the electrochemical potential scale can be correlated with energy scales of electrons in surface physics. If measuring work functions or electron affinities, the reference value is the free electron in vacuum. Mehl and Lohmann calculated for the electron affinity of a hydrogen electrode —4.5 eV using the following Bom-Haber process... 

As for iron, differences in redox potentials are relatively small between the major oxidation states of copper, that is, Cu(I) and Cu(II). This gives copper its main function as a cofactor in enzymatic reactions involving electron-transfer processes. In the human body, most of the copper (about 40%) is present in muscle tissue with significant amounts also present in the liver, brain, and skeleton. About 5% of the copper can be found in serum, of which 80-90% is present as ceruloplasmin. Ceruloplasmin in serum and hephaestin at the basolateral side of the mucosa ensure oxidation of circulating Fe to Fe for iron binding to transferrin. Unbound Fe is a major source of oxidative stress through Fenton/Haber-Weiss chemistry. Copper together with zinc is also a cofactor for superoxide dismutase, a key molecule in the anti-oxidant defense system of the body ]74]. 

Kern and other investigators [9,10] found BZ2O2 to be very effective in both aqueous and nonaqueous media with or without heavy metals as a component. Kern [9] based his theory of reaction on Haber s earlier suggestions and formulated the production of radicals as an electron transfer process. He proposed a Haber-Weiss type of mechanism for two-component systems ... 

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