ECE/DISP processes

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. 

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 ... 

The ECE/DISP framework covers many two-electron oxidation and reduction processes (38). Using a cathodic process as an example, the reduction of an initial species, A, producing B is followed by a first-order homogeneous reaction yielding an intermediate (C), which is often more easily reduced at the electrode (to give D) than the starting material. Under these conditions E°ad H km, and the second electron transfer can occur in the solution, by disproportionation of B and C, so that overall the following steps have to be considered ... 

The diffusion-limited current-solution flow rate behaviour for the reduction of fluorescein at about pH 10 was studied. The observed behaviour, shown in Fig. 33, revealed a transition from two-electron transfer at low flow rates to one-electron transfer at fast flow rates. This behaviour is characteristic of an ECE or DISP process (vide infra), where, at fast flow rates, the product of the first electron transfer is swept away from the electrode before the chemical step takes place and one-electron behaviour is observed. 

The difference between ECE and DISP mechanisms is that, for ECE, there are two heterogeneous electron transfers, but a DISP process involves a homogeneous second-electron transfer, via disproportionation. Compton described the kinetic scheme... 

Rajendran L (2000) Pade approximation of ECE and DISP processes at channel electrodes. Electrochem Commun 2 186-189... 

Leslie WM, Alden JA, Compton RG, Silk T (1996) ECE and DISP processes at channel electrodes analytical theory. J Phys Chem 100 14130-14136... 

FIGURE 5.13 Diffusional and chemical processes occurring within the tip/substrate domain in the case of (A) an ECE pathway and (B) a DISPl pathway. (Reprinted with permission from Demaille, C., Unwin, P.R., and Bard, A.J., Scanning electrochemical microscopy. 33. Application to the study of ECE/DISP reactions, J. Phys. Chem., 100, 14137-14143, 1996. Copyright 1996 American Chemical Society.)... 

Thus, in the latter case, the term ECE has to be abandoned and replaced by disp . (the SET occurs obviously via a disproportionation process). Finally, the strong base R formed after an overall two-electron reaction is protonated by the solvent or by any acidic impurity. Alternative mechanisms could be proposed taking into account that R or Ar may abstract hydrogen atoms from the solvent ... 

In the absence of radical traps, the radical R is converted immediately into the carbanion R by an ECE or a DISP mechanism, according to the distance from the electrode where it has been formed. B is a strong base (or nucleophile) that will react with any acid (or electrophile) present. Scheme 2.21 illustrates the case where a proton donor, BH, is present. The overall reduction process then amounts to a hydrogenolysis reaction with concomitant formation of a base. This is a typical example of how singleelectron-transfer electrochemistry may trigger an ionic chemistry rather than a radical chemistry. This is not always the case, and the conditions that drive the reaction in one direction or the other will be the object of a summarizing discussion at the end of this chapter (Section 2.7). 

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