Complementary oxidation-reduction reactions

Throughout this chapter, you have read about oxidation-reduction reactions. You know that redox reactions involve the loss and gain of electrons. Thus, the pairing or complementary nature of redox reactions is probably apparent to you. So, let s consider the two halves of redox reactions. 

Both FMN and FAD occur as tightly bound coenzymes (Fig. 7.15). They participate in oxidation/reduction reactions in which the riboflavin part is oxidized or reduced complementary to the reduction or oxidation of the substrate. The enzymes are called flavoproteins and the oxidized forms have an intense color (yellow, red, or green), although the reduced forms are colorless. NADH dehydrogenase is an important example of a flavoprotein other flavoproteins are involved in oxidative degradation of pyruvate, fatty acids, and amino acids. 

Kashiwazaki67 has fabricated a complementary ECD using plasma-polymerized ytterbium bis(phthalocyanine) (pp—Yb(Pc)2) and PB films on ITO with an aqueous solution of 4M KC1 as electrolyte. Blue-to-green electrochromicity was achieved in a two-electrode cell by complementing the green-to-blue color transition (on reduction) of the pp—Yb(Pc)2 film with the blue (PB)-to-colorless (PW) transition (oxidation) of the PB. A three-color display (blue, green, and red) was fabricated in a three-electrode cell in which a third electrode (ITO) was electrically connected to the PB electrode. A reduction reaction at the third electrode, as an additional counter electrode, provides adequate oxidation of the pp Yb(Pc)2 electrode, resulting in the red coloration of the pp—Yb(Pc)2 film. 

In many other cases, detailed examination of platinum(IV) substitution reactions has shown that the mechanisms involve oxidation-reduction steps. These redox reactions can be collected into two classes according to whether a bielectronic or a monoelectronic redox species reacts with the platinum complex (i.e. complementary and non-complementary redox reactions, respectively). 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

Cyclic artificial photosynthetic systems (Fig. 11) include an oxidation process complementary to the reduction reaction. For light-driven reductive syntheses of valuable chemicals or for the removal of environmental pollutants the concept of utilizing a sacrificial electron donor can be adapted. Yet, for the application of artificial photosynthetic systems as fuel generation devices, several basic criteria must be met by the complementary oxidation process ... 

The marked irreversibility of the oxygen evolution and reduction reactions in aqueous solutions has imposed severe limitations on the mechanistic information which can be obtained for both reactions. In general, at the current densities normally employed for kinetic studies, the current-potential data are insensitive to the back reaction, which normally occurs early on in the multi-step reaction sequence. Further, the reduction and oxidation processes are usually studied only at widely separated potentials. Thus, the surface conditions, whether in the case of metals or bulk oxides, probably differ sufficiently such that the reduction and oxidation pathways may not be complementary. The situation is complicated further by the large number of possible pathways for both reactions. 

As with most concepts involving electrons, oxidation and reduction reactions are often initially misinterpreted as complicated and difficult to understand. Oxidation and reduction are simply complementary processes involving the loss and gain of electrons from molecules, atoms or ions. Whereas oxidation is the loss of one or more electrons (i.e. oxidation is loss (OIL)), reduction is gain of one or more electrons (i.e. reduction is gain (RIG)). These abbreviations are an easy way to remember the difference between these two processes with respect to electron changes (OIL RIG). As these processes are complementary and occur in the same system they are often referred to as redox reactions (i.e. reduction and oxidation). Figure 4.1 provides a simple illustration of this principle. 

Oxidation and reduction are complementary processes. The oxidation half-reaction produces an electron that is the reactant for the reduction half-reaction. The combination of two half-reactions, one oxidation and one reduction, produces the complete reaction. 

Both have complementary oxidation and reduction reactions occurring at each electrode. 

An oxidation reaction cannot take place without an accompanying reduction reaction - the electrons have to go somewhere - but it is convenient to nonetheless split cell reactions into two complementary half-cell reactions. In our copper-iron case, these half-cells are... 

Photochemical reduction systems (Figure 5.11) require efficient light harvesting, usually by a so-called dye or sensitizer, and efficient charge separation and energy utilization. Transition metal complexes, particularly tris(2,2 -bipyridine)ruthenium(ll), serve as sensitizers. The overall reaction carried out must be a useful one. That is, in addition to carbon dioxide reduction, the complementary oxidation process (which provides the electrons) should be a desirable one. Both reduction and oxidation processes generally require catalysis. For carbon dioxide reduction, a number of the catalysts used in electrochemical systems are also effective in photochemical systems, as outlined below. 

Many reactions catalyzed by metalloenzymes involve electron transfer. On the simplest level, one can consider electron transfer reactions to be complementary when there are equal numbers of oxidants and reductants and the metals transfer equal numbers of electrons as shown in equation 1.25 ... 

When oxidants and reductants change their oxidation state by an equal number of units, the reaction is known as complementary reaction. When the oxidant and the reductant change their oxidation state by a different number of units, the electron transfer reaction is known as a non-complementary reaction. 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

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