Irreversible oxidation

In electrochemical oxidation of l-hydroxy-3-imidazoline-3-oxides containing one to four H atoms at a-C, one observes in ESR-spectra not only triplet splitting of the nucleus 14N of the nitroxyl group (a v 15-16 G) but also splitting of the neighboring protons (a// 18-20 G), with multiplets corresponding to their number (from doublet to quintet) (101). Unlike spatially hindered hydroxylamines which show reversibility in electrochemical oxidation, hydroxylamines with H at a-C are oxidized irreversibly. Oxidation of hydroxylamines with nitroxyl radical proceeds easily and with quantitative yields (102). In the oxidation of asymmetric polylluorinated hydroxylamines with Mn02, isomeric polyfluorinated nitrones have been obtained (103). 

Adsorption of carbon dioxide or oxygen on the praseodymium samples was carried out in the pressure range of 1-40 Pa to evaluate the number of chemisorption sites on the samples. Praseodymium oxide irreversibly adsorbed 9.5 x 10" mol g of carbon dioxide. The amount of oxygen irreversibly adsorbed on the sample was 15.2 x 10" mol g Carbon dioxide or oxygen was not adsorbed on the samples containing chlorine, i.e., praseodymium chloride and praseodymium oxychloride prepared from the chloride by heating under oxygen flow at 750°C for 1 h. 

The highly hydrogenated derivative, C60H36 displays only one reversible reduction in THF, which appears to involve two electrons [87]. Reductions are observed at —3.24 and —3.18 V vs. Fc/Fc+ when the voltammetry is conducted at +14 and — 38°C respectively. These highly negative potentials approach the value for the reduction of benzene (see Table 10), and support the notion that as fullerene double bonds become saturated, reduction becomes more difficult. No oxidation was obtained in THF, but the compound oxidizes irreversibly in DCM and MeCN at +1.22 and +1.25 V vs. Fc/Fc+ at —47 and —28 °C respectively. 

If the voltammetric scan is not reversed after the first oxidation, a second oxidation (Epa = 0.2-0.3 V) is observed (Figure 6c). This oxidation renders the lower potential oxidation irreversible (Epa = 0-0.1 V) and also leads to the observation of an irreversible reduction at negative potentials (Epc = —0.8 to —0.9 V). It has not been possible to measure accurately the number of electrons involved in either the second oxidation or in the coupled reduction process by coulometry due to the formation of films on the electrode surfaces. However, in comparison with that observed for the first oxidation, the peak currents are consistent with one- and two-electron processes, respectively. In spite of the irreversible nature of the cyclic voltammogram, continuous cycling be-... 

In THF, [Fe(C5H4PPh2)2]Mo(CO)4 undergoes a ferrocene-centered oxidation, irreversible because of fast degradation of the instantaneously electrogenerated monocation [52], This result is not at all unexpected, because the one-electron oxidation of the free disubstituted ferrocenylphosphine Fe(C5H4PPh2)2 is followed by chemical complications [52, 53]. Furthermore, theoretical arguments on the... 

Enzymatic reductive animation with NADH as the cofactor can only be operated on a large scale if the cofactor is regenerated. Wandrey and Kula have developed a regeneration scheme using formate as the reductant of NAD+ generated upon reductive animation (Fig. 15.3-1). The formate is oxidized irreversibly to CO2 by formate dehydrogenase (FDH, E.C. 1.2.1.2)1621. 

Some reactions in which one half-reaction is irreversible do occur rapidly. Several oxidizing and reducing agents containing oxygen are reduced or oxidized irreversibly but may be speeded up by addition of an appropriate catalyst. The oxidation of arsenic(ni) by cerium(IV) is slow, but it is catalyzed by a small amount of osmium tetroxide, OSO4. 

If a substance is reduced or oxidized reversibly, then its half-wave potential wiU be near the standard potential for the redox reaction. If it is reduced or oxidized irreversibly, the mechanism of electron transfer at the electrode surface involves a slow step with a high energy of activation. Therefore, extra energy must be applied to the electrode for the electrolysis to occur at an appreciable rate. This is in the form of increased applied potential and is called the activation overpotential. Therefore, Exa will be more negative than the standard potential in the case of a reduction, or if will be more positive in the case of an oxidation. An irreversible wave is more drawn out than a reversible wave. Nevertheless, an S-shaped wave is still obtained, and its diffusion current will be the same as if it were... 

In such partial systems it is inevitable that one of the components be oxidized (electron donor in the case of water reduction) or reduced (electron acceptor in cases of water oxidation) irreversibly. In principle, reversible electron donors and acceptors can be used but cross reactions (as observed... 

Despite the formally zerovalent state, the homoleptic metal cabonyls are relatively mild donors, consistent with their ionization potentials of 8.5 0.5 V (Table V). [In solution, these complexes are oxidized irreversibly.] Successive introduction of stronger donor ligands (e.g. phosphines, sulfides, and/or isonitriles) results in a decrease in IP and Eox, consistent with increasing electron density on the metal center. [Compare the IP of Mo(CO)6 (8.50 V) [53] with Mo(CO)2 (Tl2-dppe)2 (6.00 V)] [54]. These metal carbonyl derivatives, especially those derived from Cr(CO)6, Mo(CO)6 and Fe(CO)s, form sets of graded electron donors with potentials that can be varied by selection of the number and donor strength of the noncarbonyl ligands [55]. 

Autoxidation and gratuitous oxidation + irreversible binding to soil components... 

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