The ECE Process

When a second electron transfer process exists the reaction scheme for the overall electrode process becomes considerably more complicated. An example of an ECE reaction scheme, the ErevCi evErev reaction, is given in Eq. (II. 1.26), and voltammetric responses simulated for this type of process are shown in Fig. II. 1.23 a. 

Three cases for this type of reaction sequence may be considered with (i) the simplest case - c,a+/a c,b+/b related to the ErevCi ev process (vide supra), (ii) c,A+/A = c,B+/B and (iii) - c,a+/a c,b+/b, the case considered in Fig. II. 1.23. For this mechanistic scheme, a reversible voltammetric response for the A /A redox couple at - c,A+/A = V can be observed at sufficiently fast scan rates. Reducing the scan rate allows the chemical reaction step with k = 200 s to compete with mass transport in the diffusion layer and the product B is detected on the reverse scan in the form of a B /B reduction process with -Ec,b+/b = -0.1 V. The normalisation of the voltammograms with the square root of the scan rate allows the peak currents for the oxidation response to be compared as a function of the scan rate. It can be seen that the peak current increases considerably from the reversible one-electron process observed at high scan rate to the chemically irre- 

The presence of several reactants in different redox states opens up the possibility of further reaction steps consistent with disproportionation reactions. For the case of 1+/ E +/a the disproportionation step introduced in Eq. II. 1.27 is important and cannot be neglected. The mechanism for the case where the first 

Alternatively, disproportionation may occur via a bimolecular process, to give the so-called DISP2 process described in Eq. (11.1.28) [106]. The latter, more complex, mechanistic schemes can be difficult to prove and quantify unequivocally and cyclic voltammetric techniques can give only limited insight depending on the type of electrode geometry and experimental time scale used. 

It is also interesting to consider cases in which the second electron transfer occurs at a potential lower than that required for the first electron transfer. Although it may be thought that this condition would be unusual it is in fact encountered in many electrochemical systems and is of fundamental importance in catalysis. 

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In single step voltammetry, the existence of chemical reactions coupled to the charge transfer can affect the half-wave potential Ey2 and the limiting current l. For an in-depth characterization of these processes, we will study them more extensively under planar diffusion and, then, under spherical diffusion and so their characteristic steady state current potential curves. These are applicable to any electrochemical technique as previously discussed (see Sect. 2.7). In order to distinguish the different behavior of catalytic, CE, and EC mechanisms (the ECE process will be analyzed later), the boundary conditions of the three processes will be given first in a comparative way to facilitate the understanding of their similarities and differences, and then they will be analyzed and solved one by one. The first-order catalytic mechanism will be described first, because its particular reaction scheme makes it easier to study. 

Enols sterically protected at the -carbon have also been investigated. In general, the cyclic voltammetric response exhibits only the already mentioned ECE process, while the reversible benzofuran oxidation is not detected. The pertinent electrode potentials are compiled in Table 2. In the case of the enolato species, the ECE process probably proceeds through separated one-electron oxidations as those of Table 1. 

It was stated earlier that one of the principles of the explicit finite difference method was that all the various parts of the electrochemical process can be treated sequentially. Thus for systems involving homogeneous reactions each iteration involves first a diffusional part as described above and then a kinetic part. To see what form this kinetic part takes let us consider the ece process described below ... 

Hence obtain an expression for under steady-state conditions. Also, find an expression for the current-potential waveshape for the ECE process. 

Since steel corrosion is an electrochemical process. Once it occurs in a concrete structure an electrochemical measure, such as electrochemical chloride extraction ECE can stop it or slow it down to a significant extent. The ECE process is as follow ... 

The occurrence of such a mechanism is also subordinated to the value of kinetic constant k (high values of k strongly favour an ECE process, the reduction rate of R or Ar being in most cases faster than any other chemical reaction). Electrochemical potential values... 

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 ... 

In many other cases (by a change in experimental conditions, faster chemical reaction) the value of the catalytic current may be governed by the SET rate (see reaction 20). The value of k1 may be found and its variation as a function of the nature of the mediator (with several values for °j) leads by extrapolation (when k2 can be assumed to be diffusion-controlled) to the thermodynamical potential °RS02Ar which is somewhat different from the reduction potentials of overall ECE processes observed in voltammetry. 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

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. 

This sequence has been proposed as a novel approach to olefin separation and purification. Ffowever, there is disagreement about the reversibility of the reaction because it has been shown that irreversible reduction to the dianionic complex occurs through an ECE process.1082 A convenient new route to 1,2-enedithiolate complexes of Ni has been reported, which starts from the bisfhydrosulfldo) complex [Ni(dppe)(SFI)2] and various a-bromoketoues(heterocycle-C(0)CH2Br). 

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... 

The very fact that the A-to-D conversion is a downhill process implies that a chain reaction may take place in the solution, in parallel to the electrode process (Scheme 2.12). After initiation by an electron (or a hole) coming from the electrode, the propagation loop involves the conversion of B into C and the oxidation of the latter by A. When > c, the solution electron transfer is a downhill reaction, whereas for , B < , c, it is an uphill reaction. It may, nevertheless, interfere in the latter case since the entire process is pulled by the B/C reaction. As sketched in Scheme 2.10, the interference of the solution electron transfer is more important for slower B/C conversion. More precisely, the factor governing the interference of the solution electron transfer is the same as in the ECE-DISP problem discussed in Section 2.2.4 (kecPA/ (Fv/ R-T)1/2. Apparently, disconcerting phenomena take place upon interference of the solution electron transfer, such as dips in the current-potential trace when (Figure 2.25a ) and trace crossing... 

As discussed in Section 2.5.1, aryl radicals are easily reduced at the potential where they are generated. This reduction that can take place at the electrode surface (ECE) or in the solution (DISP) opposes the substitution process. This three-cornered competition between substitution (SUBST) electron + proton transfer (ECE or DISP) depends on two competition parameters that are closely similar to the HAT-ECE-DISP parameters described in the preceding section ... 

Both the rhodium atoms assume a tetrahedral geometry with respect to the RI12P2 plane (the TTD label derives from the tetrahedral-tetrahedral geometry of the two rhodium atoms in the dianion). On this basis, the overall electrode process involves the ECE mechanism illustrated in Scheme 4, where TPA = tetrahedral-planar monoanion, TTA = tetrahedral-tetrahedral monoanion. 

The membranes of the microhotplates were released by anisotropic, wet-chemical etching in KOH. In order to fabricate defined Si-islands that serve as heat spreaders of the microhotplate, an electrochemical etch stop (ECE) technique using a 4-electrode configuration was applied [109]. ECE on fully processed CMOS wafers requires, that aU reticles on the wafers are electrically interconnected to provide distributed biasing to the n-well regions and the substrate from two contact pads [1 lOj. The formation of the contact pads and the reticle interconnection requires a special photolithographic process flow in the CMOS process, but no additional non-standard processes. 

Cathodic substitution stands for C,C bond or C, heteroatom bond formation with cathodically generated anions. The question of regioselectivity is encountered in the reaction of such anions with allyl halides (path a) or in the reaction of allyl anions generated in an ECE process from allyl halides (path b). Cathodic reductive silylation of an allyl halide proceeds regioselectively at the less substituted position (Fig. 15) [91]. From the reduction potentials of the halides it is proposed that the reaction follows path b. 

Cathodic addition involves either the reduction of a 7T-system in an ECEC process (electrochemical, chemical, electrochemical, chemical) and the chemical reaction (C) of the intermediates with a carbon or heteroatom electrophile or the cathodic conversion of a C—X bond in an ECE process into an anion that adds to an electrophilic jr-system. The electrochemical Birch reduction of arenes... 

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