Chemical reversibility examples

These 3d1 complexes display a quasireversible one-electron oxidation corresponding to the chemically reversible step [VOivL]/[VOvL] + (L = Schiff base ligand). As an example, Figure 3 shows the cyclic voltammetric response of [VO(acen)].7 The relative potential values are reported in Table 3. 

Most electrode reactions of interest to the organic electrochemist involve chemical reaction steps. These are often assumed to occur in a homogeneous solution, that is, not at the electrode surface itself. They are described by the usual chemical kinetic equations, for example, first- or second-order reactions and may be reversible (chemical reversibility) or irreversible. 

An extensive database has demonstrated that many chemicals that are positive in this test also exhibit mutagenic activity in other tests. There are, however, examples of mutagenic substances, which are not detected by this test reasons for these shortcomings can be ascribed to the specific nature of the endpoint detected, differences in metabolic activation, or differences in bioavailability. On the other hand, factors which enhance the sensitivity of the bacterial reverse mutation test can lead to an overestimation of mutagenic activity. The bacterial reverse mutation test may not be appropriate for the evaluation of certain classes of chemicals for example, highly bactericidal compounds (e.g., certain antibiotics) and those which are thought (or known) to interfere specifically with the mammalian cell replication system (e.g., some topoisomerase inhibitors and some nucleoside analogues). In such cases, mammalian mutation tests may be more appropriate. 

A particularly interesting example of bin-uclear Mn electrochemistry is that reported by Holm et al. for the [Mn2(salmp)2]" (n = 2+, 1+, 0,1—, 2—) complex [88]. H3salmp is a pentadentate SB ligand that bridges both Mn centers by means of a centrally positioned phenolate group (7). [Mn2(salmp)2]" undergoes four chemically reversible one-electron transfers. 

Figure 3.31 illustrates the simplest example of LAPV at a stationary electrode. In this case we assume that the time delay td between pulses is such that the initial condition is restored. This implies that all concentration profiles are completely relaxed before the next pulse is applied and every pulse initiates a new chronoamperometric experiment. The length of td required to accomplish this will depend on the chemical reversibility of the system, being shorter for reversible reactions and longer for irreversible reactions. 

Although the one-electron reduction of nitrobenzene to its radical anion in dipolar aprotic solvents is a classical example of a chemically reversible redox couple, the reductions of many organic compounds are chemically irreversible. The redox behavior of /7-chlorobenzonitrile is typical of those systems in which the initial electrode product undergoes rapid, irreversible chemical reaction to give another reducible species. 

A molecule will rarely take up or release a large number of electrons without bond disruptions. An exceptional example involves the reduction of an Fe(III) tris-bipyridyl complex, in which six electrons are added stepwise to the complex. The chemical reversibility of the processes may be followed by cyclic voltammetry (Fig. 23.11). 

Dual-electrode LCEC is very useful for the selective detection of chemically reversible redox couples. In this case, two electrodes are placed in series (Fig. 27.1 OB). The first electrode acts as a generator to produce an electroactive species that is detected more selectively downstream at the second electrode, which is set at a more analytically useful potential. One excellent example of the use of a dual-electrode detector for electrochemical derivatization is the detection of disulfides [34]. In this case, the first electrode is used to reduce the disulfide to the corresponding thiol. The thiol is then detected by the catalytic oxidation of mercury, described earlier. Because of the favorable potential employed at the second electrode, the selectivity and sensitivity of this method are extremely high. In addition, thiols can be distinguished from disulfides by simply turning off the generator electrode. 

The two major classes of voltammetric technique 4 Evaluation of reaction mechanisms 6 General concepts of voltammetry 6 Electrodes roles and experimental considerations 8 The overall electrochemical cell experimental considerations 12 Presentation of voltammetric data 14 Faradaic and non-Faradaic currents 15 Electrode processes 17 Electron transfer 22 Homogeneous chemical kinetics 22 Electrochemical and chemical reversibility 25 Cyclic voltammetry 27 A basic description 27 Simple electron-transfer processes 29 Mechanistic examples 35... 

For example, [Re (CN)(CO)3(bpy)] in strictly aprotic solvents at low temperature is reduced in two chemically reversible one-electron steps at —1.77 and —2.42 V, which correspond to formation of [Re (CN)(CO)3(bpy)] z = 1— and 2—, respectively [130]. The first reduction of [Re(CO)4(bpy)]+ or [Re(P(OEt3))(CO)3(bpy)]+ is chemically reversible even at room temperature [133, 135]. Halide complexes [Re (X)(CO)3(bpy)] (X = Cl, Br) have a complicated reduction pattern because of further reactions of the reduced products. Several reaction intermediates and products have been characterized spectroelectrochemically [131, 135-137] and rate constants of the main steps have been determined [130] by low-temperature cyclic voltammetry. 

Endocrine disruptors are exogenous chemicals (for example, PCBs) that enter the body and act like hormones. They can dismpt physiology, sometimes with devastating consequences. Startling evidence from animal studies shows that male fish in detergent-contaminated water express female characteristics, turtles are sex-reversed by PCBs, male frogs exposed to a common herbicide form multiple ovaries, pseudohermaphroditic offspring are produced by polar bears found in contaminated waters, and seals in contaminated water have an excess of uterine fibroids.46... 

Whether or not a half-reaction exhibits chemical reversibility depends upon solution conditions and the time scale of the experiment. For example, if the nitrobenzene reaction is carried out in an acidic acetonitrile solution, the reaction will become chemically irreversible, because PhN02 reacts with protons under these conditions. Alternatively, if the reduction of ArX is studied by a technique that takes only a very short time, then the reaction can be chemically reversible in that time regime ... 

A number of experimental methods, such as impedance spectroscopy (Chapter 10), are based on the application of small perturbations to a system otherwise at equilibrium. These methods often provide the exchange current in a relatively direct manner, as long as the system is chemically reversible. It is worthwhile for us to consider the exchange properties of a multistep process at equilibrium. The example that we will take is the overall process O + R, effected by the general mechanism in (3.5.8)-(3.5.10) and... 

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