Solvent standard oxidation potential

The E value reflects the stabilization energy of the negative charge by surrounding solvent molecules. Thus its variation by solvation can be used as one of the solvent parameters. Since reaction 1 is nothing but a one-electron oxidation reaction, E can also be called optical oxidation potential The standard oxidation potential value is often diffrcult to be determined because many redox reactions are not reversible. Therefore, the E value should be a good alternative as a measure of redox reactivity for which the equilibrium redox potential is not known. 

The halogens CI2, Br2 and I2 are commercially available (cheap) common mild (but non-innocent) oxidants which are soluble in organic solvents including nonpolar ones. The order of their oxidizing powers follows that of their standard oxidation potentials E° relative to FeCp2 in MeCN CI2 (0.18 V) > Br2 (0.07 V) > I2 (-0.14 V) ... 

FIGURE3.7 The potential window for the redox chemistry of life. Redox chemistry in living cells is approximately limited by the standard potentials for reduction and oxidation of the solvent water at neutral pH. Approximate standard reduction potentials are also indicated for the commonly used oxidant ferricyanide and reductants NADH and dithionite. 

Except for very electron-rich donors that yield stable, persistent radical cations, the ox values are not generally available.64 Thus the cation radicals for most organic donors are too reactive to allow the measurement of their reversible oxidation potentials in either aqueous (or most organic) solvents by the standard techniques.65 This problem is partially alleviated by the measurement of the irreversible anodic peak potentials E that are readily obtained from the linear sweep or cyclic voltammograms (CV). Since the values of E contain contributions from kinetic terms, comparison with the values of the thermodynamic E is necessarily restricted to a series of structurally related donors,66 i.e.,... 

The characteristics of redox reactions in non-aqueous solutions were discussed in Chapter 4. Potentiometry is a powerful tool for studying redox reactions, although polarography and voltammetry are more popular. The indicator electrode is a platinum wire or other inert electrode. We can accurately determine the standard potential of a redox couple by measuring the electrode potential in the solution containing both the reduced and the oxidized forms of known concentrations. Poten-tiometric redox titrations are also useful to elucidate redox reaction mechanisms and to obtain standard redox potentials. In some solvents, the measurable potential range is much wider than in aqueous solutions and various redox reactions that are impossible in aqueous solutions are possible. 

The oxidation processes of various anions, including halide ions, have been studied by voltammetry at solid electrodes [38 a]. For example, at a platinum electrode, iodide ion (I-) in various organic solvents is oxidized in two steps, i.e. 3I -> I3 + 2e and 2Ii-> 3I2+2e. From the difference in the standard potentials of the two steps, the dissociation constants of I3, p/f=-log ([yin/KD- have been determined as in Table 8.4. In aqueous solutions, p/f is 3 and I3 is not stable enough to use this method. The electrode oxidation of CN has been studied in several solvents using ESR spectroscopy [38b]. From the ultimate formation of the relatively stable tricyanomethylenimine radical anion, the following steps have been considered ... 

At a platinum electrode that is slightly activated by platinization, the dissolved hydrogen in various solvents is oxidized nearly reversibly by H2= 2H+ + 2c. We can determine by cyclic voltammetry the standard potential of this process. If the standard potentials in various solvents are compared using a common potential scale, the Gibbs energies of transfer of H+ can be obtained [44], With electrodes other than platinum, it is difficult to observe reversible oxidation of dissolved hydrogen [44 b],... 

Marcus has introduced a model for, S N 2 reactions of the ET type based on two interacting states which takes into account the relevant bond energies, standard electrode potentials, solvent contributions, and steric effects.87 The rate constant for intramolecular electron transfer between reduced and oxidized hydrazine units in the radical cation of the tetraazahexacyclotetradecane derivative (43) and its analogues has been determined by simulation of then variable temperature ESR spectra.88 The same researchers also reported then studies of the SET processes of other polycyclic dihydrazine systems.89,90... 

Substituent or solvent effects may be similar for concerted and stepwise processes. It has been shown that provided the rates of reverse reactions are almost independent of changes in oxidation potential, plots of E°, the standard reduction potential for the half cell (8) against log kf for a series of acceptors, Ox +, reacting with a hydride donor must have a slope of 30 mV/ log unit whether the rate-limiting step is hydride transfer, or hydrogen-atom transfer, or electron transfer (Kurz and Kurz, 1978). 

Owing to its stability, solubility, and highly reproducible oxidation behavior, ferrocene has long been used as an electrochemical standard in nonaqueous solvents. Not surprisingly, the electron-donor or -acceptor properties of ring substituents in ferrocenes and other metallocenes have been repeatedly evaluated with electrochemical techniques. Measurements have been obtained using polarography,150 cyclic voltammetry (CV),151 chronopotentiometry,152 photoelectron spectroscopy, 53 and Fourier transform ion cyclotron resonance mass spectrometry.154 Extensive compilations of such data are available.155 156 Historically, variations of oxidation potentials have been discussed almost solely in terms of the... 

The two primary reference works on inorganic thermochemistry in aqueous solution are the National Bureau of Standards tables (323) and Bard, Parsons, and Jordan s revision (30) (referred to herein as Standard Potentials) of Latimer s Oxidation Potentials (195). These two works have rather little to say about free radicals. Most inorganic free radicals are transient species in aqueous solution. Assignment of thermodynamic properties to these species requires, nevertheless, that they have sufficient lifetimes to be vibrationally at equilibrium with the solvent. Such equilibration occurs rapidly enough that, on the time scale at which these species are usually observed (nanoseconds to milliseconds), it is appropriate to discuss their thermodynamics. The field is still in its infancy of the various thermodynamic parameters, experiments have primarily yielded free energies and reduction potentials. Enthalpies, entropies, molar volumes, and their derivative functions are available if at all in only a very small subset. 

The metal hexahalides MoFe, WFe, UF6 and WC are strong oxidants which are commercially available and soluble in various organic solvents, but the hexafluorides are readily hydrolyzed to the extremely corrosive and dangerous HF, and have therefore been little used. The standard redox potential of WCl6 is around 1.1 V relative to FeCp2 [222, 223] and it oxidizes [N(C6H4Br-4)3] to [N(C6H4Br-4)3][ WCle] [224]. 

In the case of charge-transfer emission (formally a bimolecular reation), Feldberg plots are also linear (e.g., [118]). The charge-transfer character of this emission, however, can be demonstrated by a correlation of emission energies of various ECL systems with the standard redox potential (e.g., correlation of the emission maxima with the oxidation potentials of the donor or with the reduction potentials of the acceptors [119]). The associated charge-transfer character can also be demonstrated by the effects of solvent and temperature on the emission energy and intensity (e.g., [120]). 

Since the reaction of RX with Mg to generate RMgX is a formal oxidative addition, then a better way to look at the reactivity is to consider the reduction potential of the RX. For the reaction to be thermodynamically feasible, AE must be positive (i.e., a —AG). This means that the reduction potential for RX must be more positive than the reduction potential of Mg " to Mg, which has a standard reduction potential of —2.375 V [34]. Since the reduction potentials are greatly affected by solvent and reference electrode, comparisons and discussion of trends must be made under the same electrochemical system (Table 2). 

---