The ECE-DISP Competition

For the ECE-DISP competition, the following equations and conditions apply. At the electrode surface (i.e., for x = 0),... 

Figure 5. Kinetic zone diagram for an ECDim/ECE/DISP competitive sequence [see text, Eqs. (48) through (53), and Table 2 for definitions of p], P2, and P3 note that for the example discussed in the text kj = kdif]. The hatched zone in b corresponds to the region without experimental validity.

Figure 7. (a) Kinetic zone diagram for the ECC/ECE/DISP competitive sequence in the ArD versus ArH formation under the experimental conditions of Fig. 6 [see text and Eqs. (48) through (51) and (58) through (61) note that for the example discussed = k if]. (b) Theoretical variations in the yield of ArD as a function of the concentration of ArX (numbers on the curves) and of the rate constants ratio ki/kn- (Experimental data from Ref. 25.)... 

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. 

In the absence of nucleophile, the aryl halide undergoes a two-electron reductive cleavage according to an ECE-DISP mechanism (Scheme 2.21). The two-electron stoichiometry occurs because the aryl radical produced on the one-electron reductive cleavage is easier to reduce than the substrate. The competition between the ECE and DISP pathways is governed by the parameter... 

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 ... 

Investigation of the competition between the ECE and DISP pathways requires considering the full partial derivative equation system involving all three species A, B, and C. In dimensionless terms,... 

This three-cornered competition may be analyzed on the basis of three two-cornered competitions, ECE-HAT, DISP-HAT, and ECE-DISP, where HAT stands for the formation of products F and H even if this type of... 

This allows the construction of the kinetic zone diagram in Fig. 7a according to the previously explained procedure. The location of the experimental systems considered in Fig. 6 indicates that the benzonitrile or naphthalene derivatives undergo an ECE/ECC competition without interference from the DISP route. Thus, as indicated by the compass card in Fig. 7a or by the formulation of Pi in Eq. (62), or 0 = 8 /D has no effect on the... 

FIGURE 2.9. ECE and DISP mechanisms in cyclic voltammetry. Dimensionless cyclic voltammograms for decreasing values of the competition parameter 2 from left to right, log 2 = 3, 1.5, —0.5, —1, — oo. 

FIGURE 2.33. ECE product (DE) and DISP product (DD) yields (Scheme 2.18) as a function of the competition parameter, a Constant concentration-contant potential and constant-current electrolyses, h Exhaustive constant-potential electrolysis. 

FIGURE 2.38. Yields of electron transfer + protonation product vs. H-atom transfer product (Scheme 2.22) in constant-potential exhaustive electrolytes as a function of the competition parameter, a ECE electron transfer b DISP electron transfer. 

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