Oxidants reduction, electron transfer

Because the breadth of chemical behavior can be bewildering in its complexity, chemists search for general ways to organize chemical reactivity patterns. Two familiar patterns are Br< )nsted acid-base (proton transfer) and oxidation-reduction (electron transfer) reactions. A related pattern of reactivity can be viewed as the donation of a pair of electrons to form a new bond. One example is the reaction between gaseous ammonia and trimethyl boron, in which the ammonia molecule uses its nonbonding pair of electrons to form a bond between nitrogen and boron ... 

SCHEME 1 Schematic illustration of the biological process of 02 dismutation into 02 and H202 catalyzed by Cu, Zn-SOD via a cyclic oxidation-reduction electron transfer mechanism. (Reprinted from [98], with permission from Elsevier.)... 

Quinones are cyclic conjugated diketones. They are colored compounds used as dyes. They also play important roles in reversible biological oxidation-reduction (electron-transfer) reactions. 

ZnO nanodisks catalyzes the dismutation of 02 to 02 and H202 via a cyclic oxidation -reduction electron transfer. Therefore, the third generation biosensor for superoxide developed. Figure 12 A shows the cyclic voltammograms of ZnO/SOD electrode in the... 

The silver-silver chloride electrode is an example of a metal electrode that participates as a member of a redox couple. The silver-silver chloride electrode consists of a silver wire or rod coated with AgCl(s) that is immersed in a chloride solution of constant activity this sets the half-cell potential. The Ag/AgCl electrode is itself considered a potentiometric electrode, as its phase boundary potential is governed by an oxidation-reduction electron transfer equilibrium reaction that occurs at the surface of the silver ... 

Oxidation-reduction (electron transfer) reactions are important in chemistry and biology. When a chemical oxidation of A by B occurs, B itself is reduced - an electron transfer process has occurred. For such chemical processes, there is always a partnership between an oxidant (which is reduced in carrying out its task) and a reductant (which is oxidized in the reaction) thus we frequently talk of oxidation-reduction, or (for ease of use) redox, reactions for what are essentially electron transfer processes. Of course, an electron can be... 

The ability of transition metals to bind and activate organic molecules, and to release the transformed organic product with turnover, forms the basis of the vast catalytic chemistry of transition metal complexes. In addition, metal atoms play a key role at the catalytic center of many enzymes. For example, metalloenzymes play key roles in hydrolysis, oxidation, reduction, electron-transfer chemistry, and many other remarkable processes such as nitrogen fixation. The long-term development of synthetic polymers that perform catalytic chemistry in a manner analogous to enzymes, is a goal of profound interest. 

The SODs are ubiquitous metallo-enzymes in oxygen-tolerant organisms and protect the organism against the toxic effects of 02 by efficiently catalyzing its dismuta-tion into O2 and H2O2 via a cyclic oxidation-reduction electron transfer mechanism as shown in Scheme 1, with Cu, Zn-SOD with an example. Two steps have been proposed in the dismutation reaction, in which the oxidized form of the metal central ions (i.e. Cu (II), Fe(III), Mn(III) for Cu, Zn-SOD, Fe-SOD and Mn-SOD, respectively) were first reduced to the reduced form (i.e. Cu (I), Fe(II), Mn(ll) for Cu, Zn-SOD, Fe-SOD and Mn-SOD, respectively) with the formation of O2 followed by a subsequent equivalent oxidation of the reduced form of the metal ions into their oxidized form by 02 with the release of H2O2 (Scheme 6.1) [98]. 

Electron donors (D) and electron acceptors (A) constitute reactant pairs that are traditionally considered with more specific connotations in mind - such as nucleophile/electrophile in bond formation, reductant/oxidant in electron transfer, base/acid in adduct production, and so on. In each case, the chemical transformation is preceded by a rapid (diffusion-controlled) association to form the 1 1 intermolecular complex9 (equation 2). 

The redox potential diagram in eq. 1 illustrates that the effect of optical excitation is to create an excited state which has enhanced properties both as an oxidant and reductant, compared to the ground state. The results of a number of experiments have illustrated that it is possible for the excited state to undergo either oxidative or reductive electron transfer quenching (2). An example of oxidative electron transfer quenching is shown in eq. 2 where the oxidant is the alkyl pyridinium ion, paraquat (3). 

Electron transfer of the glucose oxidase/polypyrrole on the electrode surface was confirmed by differential pulse voltammetiy and cyclic voltammetry. The glucose oxidase clearly exhibited both reductive and oxidative current peaks in the absence of dissolved oxygen in these voltammograms. These results indicate that electron transfer takes place from the electrode to the oxidized form of glucose oxidase and the reduced form is oxidized by electron transfer to the electrode through polypyrrole. It may be concluded that polypyrrole works as a molecular wire between the adsorbed glucose oxidase and the platinum electrode. 

For the cerium pz sandwich complex, only data for the analogous porphyrin sandwich is available. For the porphyrin, not only are one ring reduction and two ring oxidations observed, a metal-based Ce(IV)/Ce(III) reduction is observed, at a lower potential than the ring oxidation. The electron-transfer rate constant for the Ce(IV)/Ce(III) process was found to be very slow, 2.2 1.0 X 10 cm 1 because the/orbital of the cerium ion is buried between the two macrocycles making it difficult for the incoming electron to access it. For the Ce IVl pz, a similar Ce(IV/III) reduction is observed, at a lower potential than that observed for the porphyrin. 

The oxidative and reductive properties of molecules can be enhanced in the excited state. Oxidative and reductive electron transfer processes according to the following reactions ... 

The majority of the enzyme-catalyzed reactions discussed so far are oxidative ones. However, reductive electron transfer reactions take place as well. Diaphorase, xanteneoxidase, and other enzymes as well as intestinal flora, aquatic, and skin bacteria—all of them can act as electron donors. Another source of an electron is the superoxide ion. It arises after detoxification of xenobiotics, which are involved in the metabolic chain. Under the neutralizing influence of redox proteins, xenobiotics yield anion-radicals. Oxygen, which is inhaled with air, strips unpaired electrons from these anion-radicals and gives the superoxide ions (Mason and Chignell 1982). 

Have you ever wondered how a battery works You can find out how in this chapter. In Chapter 11, you learned how oxidation-reduction reactions transfer electrons from one species to another. Batteries use oxidation-reduction reactions, but they are carefully designed so the flow of electrons takes place through a conducting wire. The first battery was made in 1796 by Alessandro Volta, and batteries are commonly called voltaic cells in his honor. There are many different ways to construct a voltaic cell, but in all cases, two different chemical species must be used. The voltage of the cell depends on which species are used. 

Apart from the all-carbon backbone, poly(vinyl ester)s also exhibit a unique 1,3-diol structure (see Fig. 1). This structure is a common motif in many natural materials, e.g. carbohydrates. A number of oxidative or reductive electron transfer processes catalysed by natural redox systems are imaginable for this motif. The 1,3-diol structure is unique for a synthetic polymer and cannot be found in any other synthetic polymer class of significance. This explains the unusual biodegradation properties discussed below. 

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

Charge reversal in the electron transfer can be observed if donor sensitizers are employed. For example, photosolvolysis of cyclohexene oxide (135), may proceed through the epoxide radical anion. Analogous fragmentation from stilbene oxide and extrusion of SC>2 from benzylsulfone has been reported when amine sensitizers are employed (136). In fact, reductive electron transfer to cyclic sulfites or carbamates, eq. 49 (137),... 

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

Reduction-oxidation Redox or oxidation-reduction. The transfer of one or more electrons between a pair of atoms. The atom accepting the electrons is reduced and the electron donor is oxidized. 

DHP Mn(III) reduction to Mn(II) is conjugated with DHP membrane oxidation. No electron transfer without PVS 70... 

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