Reduction potentials for dioxygen

TABLE 9.3 Standard Reduction Potentials for Dioxygen Species in Water [02l 1 atm (1.0 mM>] (Formal Potentials for 02 at Unit Activity)... 

TABLE 9.4 Formal Reduction Potentials for Dioxygen Species in Acetonitrile [02 at 1 atm (8.1 mM>]... 

Figure 1 Standard reduction potentials for dioxygen species in water [1 atm O2, pH 7, V vs. NHE]...

Table 1 Standard reduction potentials for dioxygen species in water ...

The standard reduction potential for dioxygen to give water at 25 °C, 1 atm, and pH = 0 is E° =. 23 V. Substitution into the Nernst equation gives... 

It should be mentioned that Spasojevic et al. [57] recently determined the two-electron reduction potential of lucigenin in water as —0.14 V. As this value is close to the one-electron reduction potential of dioxygen °[02 702] = — 0.16 V, these authors regarded their finding as a support for lucigenin redox cycling. However, it has been demonstrated long ago that two-electron reduction potentials cannot be used for the calculation of equilibrium for one-electron transfer processes [58]. 

Although reduction potentials may be estimated for half-reactions, there are limits for their values that correspond to both members of a couple having stability in an aqueous system with respect to reaction with water. For example, the Na+/Na couple has a standard reduction potential of -2.71 V, but metallic sodium reduces water to dihydrogen. The reduced form of the couple (Na) is not stable in water. The standard reduction potential for the Co3 + / Co2 + couple is +1.92 V, but a solution of Co3+ slowly oxidizes water to dioxygen. In this case the oxidized form of the couple is not stable in water. The standard reduction potential for the Fe3T/Fe2+ couple is +0.771 V, and neither oxidized form or reduced form react chemically with water. They are subject to hydrolysis, but are otherwise both stable in the aqueous system. The limits for the stability of both oxidized and reduced forms of a couple are pH dependent,... 

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. 

Fio. 1. standard oxidation-reduction potentials for the steps involved in the conversion of dioxygen to water at 25° and pH 7. 

Table3.1 Standard reduction potentials for reactions of relevance to the two- and four-electron reduction pathways of dioxygen in acid and alkaline aqueous solutions.

However, benzil [PhC(O)C(O)Ph] cannot enolize and is dioxygenated by Oz - to give two benzoate ions. Scheme 7-11 outlines a proposed mechanism that is initiated by nucleophilic attack. An alternative pathway has been proposed i in which the initial step is electron transfer from 02 - to the carbonyl, followed by coupling of the benzil radical with dioxygen to give the cyclic dioxetanelike intermediate. However, the outer-sphere reduction potential for electron transfer from O2 to a carbonyl carbon is insufficient (-0.60 V versus NHE, Chapter 2). 

The reduction potential for the four-electron reduction of dioxygen (Reaction 5.1) is a measure of the great oxidizing power of the dioxygen molecule. However, the reaction involves the transfer of four electrons, a process that rarely, if ever, occurs in one concerted step, as shown in Reaction (5.2). 

The reduction potentials for O2 and various intermediate species in H2O at pH 0, 7, and 14 are summarized in Table IV similar data for -02 in MeCN at pH —8.8, 10.0, and 30.4 are presented in Table V. For those couples that involve dioxygen itself, formal potentials are given in parentheses for -02- at unit molarity ( 10 atm [ O2 ] Ri 1 mM at 1 atm partial pressure). The reduction manifolds for O2 (Tables IV and V) indicate that the limiting step (in terms of reduction potential) is the first electron transfer to 02- and that an electron source adequate for the reduction of 02- will produce all of the other reduced forms of dioxygen (O2, HOO, HOOH, HOO, HO-, H2O, H0 ) via reduction, hydrolysis, and disproportionation steps. Thus, the most effective means to activate -02 is the addition of an electron (or hydrogen atom H O -b H ), which results in significant fluxes of several reactive oxygen species. 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

It should be mentioned that in aprotic media redox potential for the reduction of superoxide to peroxide, E 02 /0 ), is significantly catodically shifted, so that it is even more negative that the redox potential for the oxidation of superoxide to dioxygen. This is exactly the reason why the superoxide is stabilized in aprotic solvents, whereas peroxide is extremely unstable under such conditions. However, coordination of superoxide to the metal center induces effect similar to that caused by protonation, and the Oj /0 redox potential shifts anodically. Thus, upon binding to a metal cation, superoxide can be reduced in aprotic media, as well. 

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