ECE process

ECE processes, with a chemical step being interposed between electron transfer steps,... 

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... 

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). 

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... 

Cyclic voltammetry (Fig. 2) and spectro-electrochemical studies (Fig. 3) in DMF have shown that the first step of the reduction of sulfur proceeds via an ECE process with a reversible chemical reaction (C) that... 

CH3CN V = 0.2 V s ) indicated that the electrode process was not a Nernstian two-electron transfer but involved two successive one-electron steps, with the second thermodynamically more favorable than the first one [32]. Therefore, the reversible, overall two-electron process in Sch. 11 is better represented by two successive, reversible, one-electron steps involving a thermodynamically unstable and undetected cation intermediate (see Sch. 13 EE process, or ECE process, where the chemical step C is a fast, reversible deformation of the M2S2 core). In agreement with this, it should be noted that the oxidation of ds-[Mo2(cp )2(/x-SMe)2(CO)4] ds-13 also... 

The imido complex [Mo2(cp)2(/r-SMe)3 (/u.-NFl)]" " 25+ undergoes an irreversible one-electron (EC) reduction [70]. Controlled potential electrolysis afforded the amido analog [Mo2(cp)2(/x-SMe)3(/x-NH2)] 26 almost quantitatively after the transfer of IF mol 25+. The amido complex was not the primary reduction product the latter was assigned as a rearranged imide radical (Sch. 18), which is able to abstract a FI-atom from the environment (supporting electrolyte, solvent, or adventitious water) on the electrolysis timescale. In the presence of protons, the reduction of 25+ became a two-electron (ECE) process. This is consistent with the protonation at the nitrogen lone pair of the primary reduction product, followed by reduction of the resulting amido cation... 

The double bond shift is explained in terms of a kinetically controlled protonation of the anion formed in an ECE process the anion would have the highest negative charge a to the ester group. Such a double bond shift seems to be rather general also for the indirect reduction of allyl alcohols.388... 

Kolbe type.413 The reaction could take place by two paths by electron removal from the n system in a conventional ECE process to give an O-71-stabilized radical or by a concerted two-electron removal from the heterocyclic it system, and C02 loss. 

Chlorobenzonitrile and adrenaline, our second example, both give electrode products that are unstable with respect to subsequent chemical reaction. Because the products of these homogeneous chemical reactions are also electroactive in the potential range of interest, the overall electrode reaction is referred to as an ECE process that is, a chemical reaction is interposed between electron transfer reactions. Adrenaline differs from/ -chlorobenzonitrile in that (1) the product of the chemical reactions, leucoadrenochrome, is more readily oxidized than the parent species, and (2) the overall rate of the chemical reactions is sufficiently slow so as to permit kinetic studies by electrochemical methods. As a final note before the experimental results are presented, the enzymic oxidation of adrenaline was known to give adrenochrome. Accordingly, the emphasis in the work described by Adams and co-workers [2] was on the preparation and study of the intermediates. 

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. 

No single working curve suffices for all first-order ECE processes. The equilibrium constant of the solution redox reaction has a marked effect upon the current-time behavior and, accordingly, an important influence on the shape of... 

Distinction among the several working curves for ECE processes is sometimes difficult, especially if data are obtained over a relatively restricted range of napp. Although the use of pen-and-ink recording limited the dynamic range in the present example, current instrumentation usually permits acquisition of results over several decades of time. The full capability of the instrumentation should be utilized in order to minimize errors in interpretation. 

Figure 21.5 (Upper left) Cyclic voltammogram of adrenaline at pH 3.0. The scan rate is 0.278 V/s. (Upper right) Working curve for this ECE process. (Lower left) Chronoamperometric data for the cyclization of adrenalinequinone at pH 3.5. (Lower right) Observed rate constant for the cyclization of adrenalinequinone as a function of pH. [From Ref. 2, reprinted with permission.]...

The overall ECE process of Equations 23.34-23.36 was implicated, with the intervening chemical reaction (Eq. 23.35) being the partial dissociation of one dppe ligand ... 

Reactions in which an electron transfer (E) is followed sequentially by a chemical reaction (C) and a second electron transfer (E), are called ECE processes. We can now see that both the top and bottom routes of eqn 9 involve ECE pathways. The top route, however, involves an oxidation as the second electrochemical step, while the bottom route is characterized by a reduction in the second stage of the electron exchange sequence. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

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. 

In the last 20 years, a big effort has been made to characterize different reaction schemes taking place at modified electrodes, with special focus on the case of biomolecules in what has been called Protein Film Voltammetry [79-83]. Among the different situations analyzed with multipulse techniques (including Cyclic Voltammetry), it can be cited the surface ECE process [89, 90] and surface reactions preceded by homogeneous chemical reactions [91]. For a more detailed revision of the different mechanisms analyzed in the case of SWV, see [19]. 

---