Dioxygen reactions, with dihydrogen

Fig. 22.5. Concentrations of components (sulfate, sulfide, carbonate, methane, and acetate) and species (O2 and H2) that make up redox couples, plotted against temperature, during a model of the mixing of fluid from a hot subsea hydrothermal vent with cold seawater. Model assumes redox couples remain in chemical disequilibrium, except between 02(aq) and H2(aq). As the mixture cools past about 38 °C, the last of the dihydrogen from the vent fluid is consumed by reaction with dioxygen in the seawater. At this point the anoxic mixture becomes oxic as dioxygen begins to accumulate.

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 interesting feature of the V-BrPO reactions with peracetic acid in the presence of amines (i.e., added amines or amine-containing buffers) is the formation of bromamines (BrNHR) [29]. Bromamines have been proposed as possible intermediates in haloperoxidase reactions [61,62] however, their direct detection had not been reported previously. Bromamine formation is not observed when the peroxide source is dihydrogen peroxide because the bromamine is rapidly reduced by dihydrogen peroxide, forming dioxygen and bromide. 

The oxidation of saturated hydrocarbons in the presence of iron- or manganese-containing catalysts can be achieved by using a variety of oxidants including alkyl hydroperoxides, peroxycarboxylic acids, iodosyl-benzene, dihydrogen peroxide, and dioxygen (9-11). It has been shown that chiral iron- and manganese-porphyrin complexes catalyze the asymmetric epoxidation of unfunctionalized alkenes (75). Except for a number of experiments in which up to 96 % enantiomeric excess (ee) has been reported (16,17), in most epoxidation reactions with chiral porphyrins only a low to moderate enantiomeric excess of the product is obtained (18,19). In association with these catalysts, alkyl hydroperoxides and iodosylbenzene are often used as primary oxidants (18,19). 

Fig. 22.6. Redox potentials (mV) of various half-cell reactions during mixing of fluid from a subsea hydrothermal vent with seawater, as a function of the temperature of the mixture. Since the model is calculated assuming 02(aq) and H2(aq) remain in equilibrium, the potential for electron acceptance by dioxygen is the same as that for donation by dihydrogen. Dotted line shows currently recognized upper temperature limit (121 °C) for microbial life in hydrothermal systems. A redox reaction is favored thermodynamically when the redox potential for the electron-donating half-cell reaction falls below that of the accepting half-reaction.

This chapter is not concerned with the thermodynamic stability of ions with respect to their formation. Rather, it is concerned with whether or not a given ion is capable of existing in aqueous solution without reacting with the solvent. Hydrolysis reactions of ions are dealt with in Chapter 3. The only reactions discussed in this section are those in which either water is oxidized to dioxygen or reduced to dihydrogen. The Nernst equation is introduced and used to outline the criteria of ionic stability. The bases of construction and interpretation of Latimer and volt-equivalent (Frost) diagrams are described. 

In this paper we discuss results obtained in constructing the organized molecular systems for PET based on the use of ultrathin lipid or surfactant membranes, and in studying the mechanisms of PET in such systems. The state of the art in conjugation of PET across membranes with catalytic reactions of water reduction to dihydrogen and its oxidation to dioxygen will be also briefly discussed. 

In the vesicle suspension of Fig. 8 it was possible to isolate the centers for dihydrogen and dioxygen evolution and thus to avoid cross reactions of S+ and A- with the catalysts for H2 and 02 evolution, respectively. However, it turned out that 02 evolution gradually inhibits the H2 evolution, because oxygen evolved in the outer volume permeates across the membranes and destroys the apparatus for dihydrogen evolution located inside the vesicles. Note, that such a problem also arises for biological systems adapted to provide simultaneous evolution of H2 and Oz [275, 276],... 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

The activation and functionalization of C-H bonds by the Pt" ion is particularly attractive because of the unusual regioselectivity, high oxidation level specificity, and mildness of reaction conditions. Moreover, Sen has recently reported that, in the presence of copper chloride at 120-160 °C, Shilov chemistry can be made catalytic with dioxygen as the ultimate oxidant [39]. A number of aliphatic acids were tested, and turnover numbers of up to 15/hour with respect to platinum were observed. H/D exchange studies also confirm the marked preference for the activation of primary C-H bonds in the presence of weaker secondary C-H bonds. This study constituted the first example of the direct use of dioxygen in the catalytic oxidation of unactivated primary C-H bonds under mild conditions that does not involve the use of a co-reductant (e. g., sacrificial metals, 2H + 2e", dihydrogen, or carbon monoxide see below). 

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