Redox reactions, categories

Another common type of reaction in aqueous solution involves a transfer of electrons between two species. Such a reaction is called an oxidation-reduction or redox reaction. Many familiar reactions fit into this category, including the reaction of metals with acid. 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

Charge transfer reactions represent an important category of electrochemical behavior. As already pointed out above, an appropriate investigation of kinetic parameters of electrochemical reactions in aqueous electrolytes suffers from the small temperature range experimentally accessible. In the following, some preliminary results using the FREECE technique are presented for the Fe2+/Fe3+ redox reaction and for hydrogen evolution at various metal electrodes. 

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

Oxidation-reduction reactions, commonly called redox reactions, are an extremely important category of reaction. Redox reactions include combustion, corrosion, respiration, photosynthesis, and the reactions occurring in batteries. 

Oxidation—reduction reactions, commonly called redox reactions, are an extremely important category of reaction. Redox reactions include combustion, corrosion, respiration, photosynthesis, and the reactions involved in electrochemical cells (batteries). The driving force involved in redox reactions is the exchange of electrons from a more active species to a less active one. You can predict the relative activities from a table of activities or a halfreaction table. Chapter 16 goes into depth about electrochemistry and redox reactions. 

Reaction 2 in Table 5.1 must qualify for an outer-sphere redox category since the bipyridine could not become detached, even by just one end of the bidentate ligand, from the inert iron(II) or iron(III) centers during the course of the rapid redox reaction. There is thus no bond breaking or making during the electron transfer, a situation making them ideal for treatment by the theoretical chemist (Sec. 5.4). 

One specific application in the first category is to estimate the time required for a reaction to reach equilibrium in nature. If equilibrium is assumed in modeling a geochemical process, it is important to know the limitations (e.g., the timescale for the assumption to be valid). For example, in acid-base reactions, the reaction is rapid and the timescale to reach equilibrium is much less than one second. Hence, pH measurement of natural waters is usually meaningful and can be used to estimate species concentrations of various pH-related reactions. However, in redox reactions, the reaction is often slow and it may take days or years to reach equilibrium. Therefore, pe (or Eh) measurement of natural waters may not mean... 

The presence of Au(I) may likewise catalyze the reaction of Au(III) with methylcob(III)alamin Wood has included this couple under his Redox Switch category. Like PtCl l-, the reaction of AuC14 with CH3B12 is enhanced by the presence of Br" (46). It remains to be proven that the Au reaction does involve methyl radical transfer, presumably with subsequent oxidation of B12r [a cob(II)alamin compound] to the observed aquocobalamin. However, the available evidence for the formation of Au(II) intermediates is more extensive and convincing (200) than for Pt(III) intermediates (201). 

A reaction is said to be enthalpy-driven if it involves a large, negative AH" with a smaller and usually unfavourable TAS at all accessible temperatures. In a thermochemical analysis of such a reaction, and in comparing several such related reactions, only the enthalpy terms need normally be considered. Most redox reactions and acid-base reactions come into this category. The latter term can be interpreted liberally to include many instances of complex formation, e.g. ... 

These reactions are part of a larger category of reactions known as redox reactions (redox is short for oxidation-reduction). Sometimes these are called displacement reactions. These are chemical reactions in which atoms of one element replace the atoms of a second element in a compound. A general equation for a single-replacement reaction involving a metal (A), replacing a metallic cation in solution (B) is ... 

For the sake of completeness, a list of the general types of synthetically useful two-electron redox reactions is given in Table 2. It should be noted that combinations of the simple categories are feasible too, making possible combined carbon-skeleton construction and functionalization in a single step. 

The mechanism of the fourth category of bimolecular surface steps is peculiar to redox reactions catalysed by metals and semiconductors. Here both reactants sit on the surface, not necessarily on adjacent sites, and the electrons are transferred from the reducing to the oxidising species through the solid catalyst. The rate therefore depends not only on the concentrations at the surface but also on the potential taken up by the catalyst, and this potential in turn is a function of the concentrations of the electroactive species present. Equations (28) and (29) fail to represent the kinetics in these cases because khel is no longer independent of concentration. These kinetics must accordingly be treated by an electrochemical method of analysis and this is done in Sect. 4.1. 

Catalytic oxidation-reduction (redox) reactions in zeolites are generally limited to reactions of molecules for which total oxidation products are desired. One important class of such reactions falls under the category of emission control catalysis. This encompasses a broad range of potential reactions and applications for zeolite catalysts. As potential catalysts one may consider the entire spectrum of zeolitic structural types combined with the broad range of base exchange cations which are known to carry out redox reactions. 

Electron transfer reactions can be divided into inner-sphere and outer-sphere categories (131). The former involve a direct interaction between the electron donor and acceptor centers via a shared ligand, whereas in the latter type of reaction the coordination spheres remain separate. Some of the redox reactions involved in forming the nonheme iron core, and in mobilizing iron from it with small reductants, may involve inner-sphere reactions, but our concern here is to consider outer-sphere reactions of ferritins. 

As a rule, within the bioprecursor category of prodrugs, non-redox reactions are infrequent. An example is found in the in vivo generation of L-cysteine from its cyclic thiocarbamate. 

Coulometry is the name given to a group of other techniques that determine an analyte by measuring the amount of electricity consumed in a redox reaction. There are two categories referred as potentiostatic coulometry and amperostatic coulometry. The development of amperometric sensors, of which some are specific for chromatographic detection, open new areas of application for this battery of techniques. Combining coulometry with the well known Karl Fischer titration provides a reliable technique for the determination of low concentrations of water. 

The reactions of transition-metal complexes with polynucleotides generally fall into two categories (i) those involving a redox reaction of the metal complex that mediates oxidation of the nucleic acid and (ii) those involving coordination of the metal center to the sugar-phosphate backbone so as to mediate hydrolysis of the polymer. Both redox and hydrolytic reactions of metal complexes with nucleic acids have been exploited with much success in the development of tools for molecular biology. 

Unlike radical cations, the quantum of chemistry originating from PET-gener-ated radical anions is still limited possibly due to the impending development of a suitable photosystem to initiate photosensitized one-elecron redox reactions in wide array of functionalities. Nevertheless, the radical anion chemistry follows, more or less, the analogous pattern of bond dissociation and addition (electrophilic/radical) reactions as observed for the radical cations. As there are not many examples to describe the separate categories, this section is subdivided... 

These redox reactions may be placed in one of three categories ... 

Electron transfer reactions. These reactions are most commonly called oxidation-reduction reactions, but if you remember the category as electron transfer you will know that in every redox reaction one species gives up electrons (is oxidized), which are then accepted by another species (is reduced). A familiar example of this is the rusting of iron by moist air. In this reaction the iron is converted to Ee and the oxygen in the air is reduced to the oxide ion,. ... 

Zeolites and related microporous materials offer abundant chemical diversity. Over the past three decades, many new synthetic zeolites have been discovered. The Structure Commission of The International Zeolite Association have approved 145 framework types (November 2003). In addition, there are numerous other zeolites with structures that are not yet known or are only hypothetical. The goals of this article are to briefly mention the main categories and principal industrial uses of zeolites, before providing more detailed discussions of molecular sieving and redox reactions. 

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