Oxidations and reductions in water

Oxidation of organic compounds has probably been the most widely investigated process because it is of interest to both academic scientists and industrial technicians. Many oxidants and catalysts are known and a number of reaction conditions have been carefully investigated [1], The modem chemical industry requires selective highly efficient oxidations and environmentally sound technological processes. One way to attain these objectives is to make a rational selection of the reaction medium. 

The use of water as reaction medium is not new. The oxidation of arenes with KMn04 in alkaline aqueous medium is a very old reaction [2]. Also, H2O2 in water, in the presence or absence of catalyst, is a more frequently used oxidant owing to its peculiarities, such as low relative molecular mass, stability and solubility in many organic solvents. It is economical and environment-friendly since it forms water as a secondary product [3]. 

In the following sections, some recent and innovative oxidations by chemical reagents in aqueous media are illustrated and classified according to the type of bond or chemical functionality involved. Enzymic oxidations in water have been widely investigated during the last decade and significant synthetic applications are known. However, this subject will be not discussed here. 

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The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

Figure 1 Potential versus pH diagram for the vanadium-water system at 25 °C. The dashed lines indicate the domains of relative predominance of the dissolved forms of the metal, but the various dissolved forms for each oxidation state are not explicit. The solid lines correspond to saturated solutions with a total vanadium concentration of 0,51 gdm-3. The long dashed lines correspond to oxidation and reduction of water (for E° values of 1.23 and 0.00 V respectively) (adapted from E. Deitombe, N. Zoubov and M. Pourbaix, in Atlas d Equilibres Electrochimiques , ed. M. Pourbaix,...

The potentials are referred to the normal hydrogen electrode (NHE). The energy levels for the oxidation and reduction of water at pH 7 are shown by horizontal lines. Energy scale in volts. 

Second, in order to involve the oxidant and reductant in the multielectron reactions of water oxidation and reduction, one has to introduce appropriate catalysts into the aqueous phases separated by the membrane. 

In aqueous or aqueous-organic SSE s the accessible potential range is dependent on the electrochemical oxidation and reduction of water (or hydroxyl ions and protons in acid or alkaline media) with formation of oxygen and hydrogen, respectively. The potentials at which these processes take place are different for different electrode materials 9 in that the anodic limit for aqueous systems... 

Water can undergo both oxidation and reduction. In the latter role, water can serve as an electron sink to any metal listed above it. These metals are all thermodynamically unstable in the presence of water. A spectacular example of this is the action of water on metallic sodium. 

Values for pe°(W) apply to the electron activity for unit activities of oxidant and reductant in neutral water, that is, at pH = 7.0 for 25°C. 

The principles of photochemical water splitting can be extended to the design of systems using photocatalytic semiconductors in the form of particles or powders suspended in aqueous solutions (Bard, 1979, 1980). In this system, each photocatalyst particle functions as a microphotoelectrode performing both oxidation and reduction of water on its surface (Figure 4). 

To improve the efficiency of photocatalysts, developments in the future must be based on an understanding of the sophisticated factors that determine the photoactivity of the water-splitting reaction (i) molecular reaction mechanisms involved in the oxidation and reduction of water on photocatalyst surfaces, (ii) structure and defect chemistry of photocatalyst surfaces, and (iii) charge transfer mechanisms between... 

The actual reduction of aqueous species is limited by the potentials corresponding to the oxidation and reduction of water, respectively, given by E = 1.23 - 0.059 pH and E = —0.059 pH, as discussed in Section 2.3.4. However, thermodynamics and kinetics do not always coincide, and a more re-... 

Aleem M. I. H., Hoch G. E., and Vamer J. E. (1965) Water as the source of oxidant and reductant in bacterial chemosyn-thesis. Biochemistry 54, 869-873. 

Table 1 Oxidants and reductants in shallow-water sediments ...

The PTEF can also describe the energy efficiency in a process where there is simultaneous oxidation and reduction of water contaminants. In such a process, the Qused of the PTEF, represents the additive contribution of the enthalpy requirements to sustain both the reduction and oxidation processes. Consistently with this view, the numerator of the PTEF shall include the addition of energy used for the oxidation and reduction processes ... 

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