Electron transfer ECE mechanism

In comparison, the level of detail in the understanding of radical ion reaction mechanisms is much lower for a number of reasons. Due to the inherently complex nature of the electron transfer-chemical reaction-electron transfer (ECE) mechanism, measurement of substituent, solvent and isotope effects will usually provide a combination of effects on all the steps involved. Introducing a donor substituent on a substrate will, for example, not only change the relative stability of the transition structures and intermediates with localized charges, but will also affect the rate constant of electron transfer and self-exchange between two substrates as well as the rate of back electron transfer. 

In acetonitrile HOOH is oxidized to O2 via an electron-transfer/chemical/ electron-transfer (ECE) mechanism... 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

The electrochemical reduction proceeds at the carbene ligand as a two-electron irreversible process. Since a single reduction wave is observed, the first reduction step must be followed by a very fast chemical reaction yielding the intermediate (probably of radicalic nature), which is reduced even more easily than the starting compound to a final product (the so-called electron transfer, chemical reaction, electron transfer (ECE) mechanism). To understand the mechanism more deeply, the products of electrochemical reduction were separated during the preparative electrolysis from the solution using a continuous extraction to hexane, isolated, and analyzed. 

The competition between heterogeneous electron transfer (eCe) and electron transfer in solution (eCeh) in the second e step (Scheme 2) and the possibility of distinguishing between these two pathways have been analyzed in detail [207,208]. It was concluded that the eCeh pathway dominates over the eCe pathway unless the measurement time is kept below approximately 10 s. The application of chronoamperometry to determine the rate constants in more complicated reaction schemes, such as, for example, the eCeCe-type mechanism, has been addressed as well [209]. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Multiple electron transfer with intervening chemical reaction—ECE mechanism... 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

The cleavage mechanism can be clarified by cyclic voltammetries as shown in Figure 5. In aprotic solution (curves a) steps (l) and (2) correspond to the successive electron transfers leading finally to the dianion. On the other hand, in protic solution (curve c), step (2) has disappeared while step (l) has grown and then obviously corresponds to an ECE process. Anyhow, and whatever the medium, step (3) is identified as that in which the produced olefin (here 1,1-diphenylethylene) is reduced in all cases. 

It should be noted that the conditions that make possible the occurrence of an ECE mechanism, involving the reduction of C at the electrode surface, involve the possibility of another mechanism in which the second electron is transferred from B to C rather than from the electrode as pictured in Scheme 2.5. This homogeneous electron transfer reaction may be viewed as a disproportionation reaction insofar that A has one oxidation number more than B and C, and D, one oxidation number less. 

Since the occurrence of the ECE mechanism implies that yD , B, it follows that Kd 1, meaning that the disproportionation reaction is strongly exergonic. Since we have assumed that the two electrode electron transfer reactions are fast, the same is true for the disproportionation... 

Two-Electron Catalytic Reactions In a number of circumstances, the intermediate C formed upon transformation of the transient species B is easily reduced (for a reductive process, and vice versa for an oxidative process) by the active form of the mediator, Q. This mechanism is the exact counterpart of the ECE mechanism (Section 2.2.2) changing electron transfers at the electrode into homogeneous electron transfers from Q, as depicted in Scheme 2.9. In most practical circumstances both intermediates B and C obey the steady-state approximation. It follows that the current is equal to what it would be for the corresponding EC mechanism with a... 

When pc —> oo, the catalytic loop is complete. The reaction sequence and the current-potential responses are the same as in the two-electron ECE homogeneous catalytic mechanism analyzed in the preceding subsection. When pc —> 0, deactivation prevails, and if the first electron transfer and the deactivation steps are fast, the same irreversible current-potential responses are obtained as in a standard EC mechanism. 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

A Chemical Reaction Interposed Between Two Electron Transfers. An electrochemical process in which the product of the electron transfer undergoes a chemical reaction that generates a species which in turn is electrochemically active is defined as an ECE mechanism. It is commonly schematized as ... 

Standard potential of the second electron transfer more anodic than that of the first electron transfer (AE01 positive). The case in which the product Ox, generated by the chemical reaction following the first electron transfer, is more easily reduced than the original species Ox constitutes another common ECE mechanism in inorganic electrochemistry. 

Electrocatalysis. There is a particular type of ECE electrode reaction mechanism which is designated as liCE (the arrows indicate that the second electron transfer consists of a inverse reaction with respect to the first). This process is called electrocatalysis and is of importance in inorganic chemistry.13... 

Quite often the electrode process would be an ECE(C) reaction, in which the second electron transfer could be a heterogeneous electron transfer from the electrode to the substrate, in which case the reaction scheme is the classical ECE mechanism [Eqs. (4), (8), and (9)], or the electron transfer could be a homogeneous reaction with AT as electron donor, the so-called DISP... 

For complex mechanisms such as ECE or other schemes involving at least two electron transfer steps with interposed chemical reactions, double electrodes offer a unique probe for the determination of kinetic parameters. Convection from upstream to downstream electrodes allows the study of fast homogeneous processes. The general reaction scheme for an ECE mechanism can be written... 

The behavior of it1/2 as a function of time can be influenced substantially by the presence of chemical reactions that are coupled to the electrode process (see Chap. 2). Consequently, characteristic variations of it1/2 versus t have been effectively utilized for the quantitative study of such homogeneous chemical reactions. The ECE reaction in which a chemical step exists between two electron transfer steps is one mechanism that has been investigated by means of chronoamperometry ... 

Kinetic studies of ECE processes (sometimes called a DISP mechanism when the second electron transfer occurs in bulk solution) [3] are often best performed using a constant-potential technique such as chronoamperometry. The advantages of this method include (1) relative freedom from double-layer and uncompensated iR effects, and (2) a new value of the rate constant each time the current is sampled. However, unlike certain large-amplitude relaxation techniques, an accurately known, diffusion-controlled value of it1/2/CA is required of each solution before a determination of the rate constant can be made. In the present case, diffusion-controlled values of it1/2/CA corresponding to n = 2 and n = 4 are obtained in strongly acidic media (i.e., when kt can be made small) and in solutions of intermediate pH (i.e., when kt can be made large), respectively. The experimental rate constant is then determined from a dimensionless working curve for the proposed reaction scheme in which the apparent value of n (napp) is plotted as a function of kt. 

This chapter concerns the study of electrode reaction mechanisms of inorganic and organometallic complexes. The emphasis is on proper use of experimental measurables from cyclic voltammetry for diagnosis of common mechanisms such as E, EC, CE, and ECE reactions. We employ the standard designation of electron transfer (et) reactions as E, and other chemical reactions as C. In practice, mechanistic studies make use of an array of electrochemical and other physical and chemical methods, but space limitations restrict our attention to the powerful and versatile technique of cyclic voltammetry (CV). If necessary, the reader may review the fundamentals of this technique in Chapter 3. 

Nicholson and Shain [24] have shown that when the same number of electrons is transferred in each step of the ECE mechanism, Equation 23.33 holds, in which Xc and XD are the current functions in the presence and absence, respectively, of the intervening chemical reaction (XD is the hypothetical value of the current function under diffusion-controlled conditions, i.e., if the C step were not present) ... 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

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