The ECE and DISP Mechanisms

Their most distinctive feature compared to the EC case is the passage from a one-electron stoichiometry to a two-electron stoichiometry as X 

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. 

COUPLING OF ELECTRODE ELECTRON TRANSFERS WITH CHEMICAL REACTIONS 

FIGURE 2.10. ECE mechanism. Variations potential (b) with the kinetic parameter, A. 

Under these conditions, the wave is exactly twice the wave pertaining to the corresponding EC mechanism, with the and the same peak potential same peak width [equations (2.6) and (2.7)]. 

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Once a DISP mechanism has been recognized, the procedures for determining the rate constant of the follow-up reaction and the standard potential of the A/B couple from peak current and/or peak potential measurements are along the same lines as the procedures described above for the ECE mechanism. A distinction between the ECE and DISP mechanisms cannot be made when the pure kinetic conditions are achieved since the peak height, peak width, and variations of the peak potential with the scan rate and rate constant are the same, and so is its independence vis-a-vis the concentration of substrate. The only difference is then the absolute location of the peak, which cannot be checked, however, unless the standard potential of the A/B couple and the follow-up rate constant are known a priori. 

The ECE and DISP mechanisms, which describe overall two-electron processes, are expressed by... 

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... 

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... 

Fig. 10.6 — A comparison of absorbance-time curves for a potential step followed by open circuit relaxation for the ece and disp 1 mechanisms. Reproduced with permission from A. Bewick, J. M. Mellor, B. S. Pons, Hlectrochem. Acta, 23, (1978), 77.

For ECE or DISP mechanisms, the parameter usually measured as a function of the convective mass transport parameter is the effective number of electrons transferred, N ff, which for two single-electron transfer steps varies between one and two as described above. 

Figure 12.3.35 Simplified zone diagram showing where different limiting ECE and DISP cases apply in terms of the parameters K and p = k C% /[(/cf + k f RT Fv) [FromC. P. Andrieux and J.-M. Saveant, in Investigation of Rates and Mechanisms of Reactions, Part II, 4th ed., C. F. Bemasconi, Ed., Wiley-Interscience, New York, 1986, with permission.]...

In either the ECE or DISP 1 case, the generation of FH" is rate-limiting. The current is therefore controlled by the transport of F to the surface and its protonation, but not by the exact mechanism by which the second electron is transferred. This is because in a potential sweep experiment it is impossible to probe the potential region between the reduction potential of FH and the reduction potential of F, under conditions where FH is present in solution. 

The fact that the normalized current ratio becomes negative at intermediate values of X with the ECE mechanism and not with the DISP mechanism stems from the same phenomenon as the one causing the tracecrossing behavior in cyclic voltammetry (Figure 2.9) (i.e., continuation of the reduction of C during the anodic scan). 

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). 

Figure 3.29b). Unlike the reaction with the iron(O) porphyrin, the electron stoichiometry is of two electrons per molecule. The alkyl iron(III) porphyrin, now formed is indeed easier to reduced than the starting iron(II) porphyrin, thus giving rise to an ECE-DISP mechanism. The rate constant may again be derived from the loss of reversibility or from the positive shift of the wave when it has become totally irreversible, and also, this time, from the passage from a two- to a one-electron stoichiometry upon raising the scan rate (see Section 2.2.2). 

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... 

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. 

There are three types of DIM 2 mechanisms. In DIM 2-ECE, the dimerization reaction is the chemical step separating two electrode processes while in the DIM 2-DISP 1 and DIM 2-DISP 2 dimers are formed in the disproportionating reactions that follow the electrode process. 

Depending on the relative values of Eh, and ki, the voltammograms associated with an ECE mechanism consists of two resolved one-electron transfer processes (Fig. 12a) or a single overall two-electron transfer process (Fig. 12b) or intermediate situations. In the above oxidative mechanism D could be formed by the homogeneous disproportionation (DISP) step (24)... 

For many mechanisms, the steady-state Eia or N tt value is a function of just one or two dimensionless parameters. If simulations are used to generate the working curve (or surface) to a sufficiently high resolution, the experimental response may be interpolated for intermediate values without the need for further simulation. A free data analysis service has been set up (Alden and Compton, 1998) via the World-Wide-Web (htttp //physchem.ox.ac.uk 8000/wwwda/) based on this method. As new simulations are developed (e.g. for wall jet electrodes), the appropriate working surfaces are simulated and added to the system. It currently supports spherical, microdisc, rotating disc, channel and channel microband electrodes at which E, EC, EC2, ECE, EC2E, DISP 1, DISP 2 and EC processes may be analysed. 

Whether this second electron transfer takes place at the electrode (ECE mechanism) or in solution (DISP mechanism) is a question of how close to the electrode the radicals are formed. This again is related to the rate of the chemical reactions, Eq. (2), (3), or (5), by which they are formed. The distinction between these types of electron transfer is important when voltammetric curves are used for kinetics and mechanism analysis. This problem is discussed in more detail in Chapters 1 and 2. 

It is not simple to distinguish between this case, where the second electron transfer occurs in bulk solution [sometimes called the DISP mechanism], and the true ECE case where the second electron transfer occurs at the electrode surface (12). 

Reaction (12.3.44) is included in the scheme because species B is capable of reducing species C in a homogeneous reaction near the electrode surface. With AE 180 mV, (12.3.44) can be taken as irreversible to the right. Because species B and C are at the same oxidation level, this reaction can be considered to be a disproportionation reaction, and ECE-schemes that include it are denoted ECE/DISP mechanisms. 

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