Reductive and Oxidative Reactions

Glycohc acid also undergoes reduction or hydrogenation with certain metals to form acetic acid, and oxidation by hydrogen peroxide ia the presence of ferrous salts to form glyoxylic acid [298-12A], HCOCOOH, and ia the presence of ferric salts ia neutral solution to form oxaHc acid, HOOCCOOH formic acid, HCOOH and Hberate CO2 and H2O. These reduction and oxidation reactions are not commercially significant. 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

The reductive transformation of arene carboxylates to the corresponding aldehydes under aerobic conditions has already been noted. In addition, aromatic aldehydes may undergo both reductive and oxidative reactions, with the possibility of decarboxylation of the carboxylic acid formed ... 

The cycloaddition, reduction and oxidation reactions emanating from a,/J-unsatu-rated nitroalkenes provide easy access to a vast array of functionalities that include nitroalkanes, N-substituted hydroxylamines, amines, ketones, oximes, and a-substi-tuted oximes and ketones [73-75], Consequently, there are numerous possibilities of using these in situ generated nitroalkenes for the preparation of valuable building blocks and synthetic precursors. 

Chapters 2 through 6 introduced many asymmetric organic reactions catalyzed by small molecules, such as C-C bond formation, reduction, and oxidation reactions. Chapter 7 provided further examples of how asymmetric reactions are used in organic synthesis. This chapter starts with a general introduction to enzyme-catalyzed asymmetric organic reactions. 

The later sections of the book deal with the actual laboratory use of catalysts for asymmetric reduction and oxidation reactions. Most of the protocols describe non-natural catalysts principally because many of the corresponding biological procedures were featured in the sister volume Preparative Biotransformations. As in this earlier book, we have spelt out the procedures in great detail, giving where necessary, helpful tips and, where appropriate, clear warnings of toxicity, fire hazards, etc. 

Consider the following pair of electrochemical reduction and oxidation reactions... 

Reduction and oxidation reactions in the subsurface environment lead to transformation of organic and inorganic contaminants. We consider chromium (Cr) as an example of an inorganic toxic chemical for which both oxidation and reduction processes may transform the valence of this element, in subsurface aqueous solutions, as a function of the local chemistry. 

Present in foodstuffs and various pharmacological and cosmetics products, organic peroxides are also generated during lipid peroxidation and prostaglandin biosynthesis. They readily undergo one-electron reduction and oxidation reactions, with the formation of alkoxyl and peroxyl radicals, respectively ... 

The major chemical processes in radiation chemistry are reduction and oxidation reactions, according to the following examples. In the gas phase, ionization predominates... 

Reduction and oxidation reactions. These are electron transfer reactions which can be either inter- or intramolecular, as in the following example. 

Figure 4.11 explains how the loaded metal can act as catalyst for both the reductive and oxidative reactions. The essential point is that the barrier height at the metal/semiconductor interface is changeable by the principle of Fig. 4 10 or other mechanisms. Thus, when the band bending is weak, photoexcited electrons mostly enter the metal and the metal acts as a catalyst for a reductive reaction (Fig. 4.11(A)). On the other hand, when the band bending is strong, the holes in the valence band mostly enter the metal and the metal acts as a catalyst for an oxidative reaction (Fig. 4.11(B)). The prevalence of one over the other depends on the magnitude of the band bending, i.e., on the relative rates of reaction of the electrons and holes at the metal-free semiconductor surface.

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