Electrode Electron Transfers with Homogeneous Chemical Reactions

COUPLING OF ELECTRODE ELECTRON TRANSFERS WITH HOMOGENEOUS CHEMICAL REACTIONS 

This chapter is devoted to electrochemical processes in which chemical reactions accompany the initial transfer of one electron. This is actually a pretty common situation with organic reactants since the radical or ion-radical species resulting from this initial step is very often chemically unstable. Although less frequent, such reactions also occur with coordination complexes, ligand exchange being a typical example of reactions that may accompany a change in the metal oxidation number. 

A first type of reaction that may affect the first electron transfer intermediate is its reduction (or oxidation) at the electrode. In most cases, the second electron transfer is energetically more costly than the first (for a discussion of exceptions to this rule, see Section 1.5). The two processes thus occur at successive values of the electrode potential. There is therefore no difficulty in preventing the occurrence of the second reaction by an appropriate adjustment of the electrode potential. 

At the level of the first electrode process, the reactions affecting the initially formed intermediate fall into two categories. One encompasses 

Elements of Molecular and Biomolecular Electrochemistry An Electrochemical Approach to Electron Transfer Chemistry, By Jean-Michel Saveant Copyright 2006 John Wiley Sons, Inc. 

---

In this form of voltammetry, the concentration distributions of each species in the electrode reaction mechanism are temporally invariant at each applied potential. This condition applies to a good approximation despite various processes still occurring such as mass transport (e.g. diffusion), heterogeneous electron transfer and homogeneous chemical processes. Theoretically it takes an infinite time to reach the steady state. Thus, in a practical sense steady-state voltammetric experiments are conducted under conditions that approach sufficiently close to the true steady state that the experimental uncertainty of the steady-state value of the parameter being probed (e.g. electrode current) is greater than that associated with not fully reaching the steady state. The... 

This section concerns heterogeneous electron transfer reactions coupled with homogeneous chemical reactions in which either the electroactive species A or the product of the electron transfer B participate as reactants. Perturbations of electrochemical responses of different techniques evoked by these reactions enable the elucidation of the mechaism and the evaluation of the kinetic parameters of the chemical steps. Chemical reactions that are indicated in the electrochemical way occur in the thin layer reaction layer) adjacent to the electrode surface only. This is illustrated in Fig. 1 where the concentration dependence of the product B on the distance from the electrode plane (with and without follow-up chemical reaction) is plotted. It must be stressed that the kinetics and the electrode mechanism are affected not only by the nature of the electroactive as well as electroinactive species including the type of the solvent, but also by the electrode material and substances adsorbed on the electrode surface. 

Firstly, we should define the types of complexity which need to be considered when dealing with homogeneous chemical reactions coupled to electron transfer. The most common one is that the conversion of primary intermediates into final product is, in fact, a sequence of several, maybe four or five, elementary steps. In addition to defining the reaction pathway, it is necessary to decide which step is the rate determining one and also to consider the possibility that two steps have approximately the same rate, or that the r.d.s. changes, say with concentration of electroactive species. It is, however, also common in organic electrochemistry to find that the electrode reaction leads to a mixture of products and this is a clear indication of a branch mechanism where two competing reactions have comparable rates branch mechanisms can even lead to the same product. A further uncertainty arises as to the source of electrons does the second... 

It is possible that the species Red generated at the electrode surface may be unstable and tend to decompose. It may also be involved in chemical reactions with other species present in solution while it is moving towards the mass of the solution (homogeneous chemical reactions) or while it is still adsorbed on the electrode surface (heterogeneous chemical reactions). Furthermore, the new species formed during such reactions may be electroactive. These kind of reactions are called following chemical reactions (following, obviously, the electron transfer). 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

The electron transfer agent, often also called mediator, which is inserted between electrode and substrate performs a homogeneous chemical reaction with the... 

For complex mechanisms such as ECE or other schemes involving at least two electron transfer steps with interposed chemical reactions, double electrodes offer a unique probe for the determination of kinetic parameters. Convection from upstream to downstream electrodes allows the study of fast homogeneous processes. The general reaction scheme for an ECE mechanism can be written... 

One of the most intriguing aspects of electrochemistry involves the homogeneous chemical reactions that often accompany heterogeneous electron-transfer processes occurring at the electrode-solution interface. The addition or removal of an electron from a molecule generates a new redox state, which can be chemically reactive. A variety of mechanisms, some of which involve complicated sequences of electrode and chemical reactions, have been characterized. Several of the more common mechanisms with examples of applicable chemical systems are described next. More examples are given in Chaps. 21 and 23. 

SEV is an effective means of probing homogeneous chemical reactions that are coupled to electrode reactions, especially when it is extended to cyclic voltammetry as described in the next section. Considerable information can be obtained from the dependence of ip and Ep on the rate of potential scan. Figure 3.20 illustrates the behavior of ip and Ep with variation in scan rate for a reversible heterogeneous electron transfer reaction that is coupled to various types of homogeneous chemical reactions. The current function j/p is proportional to ip according to the equation... 

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. 

Multistep electrode reactions can also be complicated by homogeneous chemical reactions. The most studied case is that the product of a first electron transfer undergoes a homogeneous chemical transformation with an electro-inactive species present in a large excess under these conditions, the reaction scheme is that corresponding to a pseudo-first-order ECE mechanism given by... 

In the former case (second electron transfer at the electrode), the overall sequence is termed ECE it is designated a DISP (rather than an ECDisp) mechanism in the second case because the homogeneous electron transfer step is close to being a disproportionation reaction. Indeed, the latter involves an electron transfer between two chemically related molecules with the same oxidation numbers. In practice the distinction between the two mechanisms is nearly impossible on experimental grounds [96] because their rate-determining step, the B—reaction is identical. 

A systematic description of all possible combinations of homogeneous chemical processes coupled to electron transfer at an electrode surface is impossible because an infinite range of theoretically possible reaction schemes can be constructed. Unfortunately, a consistent form of nomenclature for defining the possible web of reaction pathways has not yet been invented. However, the lUPAC nomenclature [89] is of assistance with respect to simple reaction schemes. In this article, the commonly employed descriptors for electron transfer (E) and chemical (C) sequences of reaction steps, e.g. ECEC, will be used for a sequence of reactions involving electron transfer-chemical process-electron transfer-chemical process. Reaction schemes involving branching of a reaction pathway will be considered later. 

The use of well-known model systems has been undertaken to confirm the validity and define the limits of the treatment of sonovoltammetry in line with the model of a uniformly accessible electrode and also to assess the influence of ultrasound on homogeneous chemical reactions coupled to the electron transfer. The uniformly accessible electrode model allows the introduction of a reaction layer, which has also been successfully employed for rotating disc voltammetry, in studies using channel electrodes [64] and in a slightly more complex form for studies in turbulent voltammetry [65]. 

There are four important modes of coupling the homogeneous chemical reaction (represented by the symbol C in the electrode mechanism) and the electron transfer (denoted by E). In all schemes A represents the electroactive substance, B the product of electron transfer. The species denoted as X, Y, Z and D, respectively, are electroinactive at the potentials of the electrode reaction. It is assumed that the substance Z reacts with the product B under regeneration of the electroactive compound A. If necessary, the reversibility of the reactions is expressed by suffixes to the symbols (e.g., C v> Eim tc). 

Mechanistic generality. The program CVSIM uses a modular structure with a general solution of the homogeneous chemical kinetics. This means that the user can simulate virtually any electrochemical mechanism that can be formulated as a combination of electron transfers at the electrode and homogeneous chemical reactions. Diffusion coefficients for each species can be specified. 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

---