Reaction Pathways and Kinetics of Redox Reactions

As discussed in Section 14.2, the oxidative or reductive transformation of an organic compound commonly requires two electrons (or, more generally, an even number of electrons) to be transferred to yield a stable product. In many cases, however, the two electrons are transferred in sequential steps (Eberson, 1987). With the transfer of the first electron, a radical species is formed which, in general, is much more reactive than the parent compound. Hence, the overall transformation rate will often be determined by the rate of transfer of the first electron from or to the organic compound. Therefore, we should be particularly interested in those compound-specific properties that are relevant for this first one-electron reaction. 

In a very simple way, we may picture a one-electron transfer reaction (e.g., the transfer of the first electron from a reductant R to an organic compound (e.g., an organic pollutant P) schematically as  

(b) by an iron porphyrin (Fe(II)porph) in aqueous solution in the presence of cysteine as bulk electron donor (Schwarzenbach et al., 1990), and (c) in an iron-reducing (ferrogenic) water-saturated aquifer column (Heijman et al., 1995). 

Note that in this context, one often speaks of an inner-sphere mechanism if there is a strong electronic coupling between R and P in the transition state, and conversely, of an outer-sphere mechanism, if the interaction is weak (Eberson, 1987). 

From Eq. 14-30 we see that we may divide a one-electron transfer into various steps (maybe somewhat artificially). First, a precursor complex (PR) has to be formed that is, the reactants have to meet and interact. Hence, electronic as well as steric factors determine the rate and extent at which this precursor complex formation occurs. Furthermore, in many cases, redox reactions take place at surfaces, and therefore, the sorption behavior of the compound may also be important for determining the rate of transformation. In the next step, the actual electron transfer between P and R occurs. The activation energy required to allow this electron transfer to happen depends strongly on the willingness of the two reactants to lose and gain, respectively, an electron. Finally, in the last steps of reaction sequence Eq. 14-30, a successor complex may be postulated which decays into the products. 

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There are multiple possible current pathways through a DSSC, as shown in Fig. 1, because the nanoporous cell consists of two interpenetrating, bicontinuous chemical phases. The relative conductivity of these two phases and of the connection between them, Rct, depends on the illumination intensity, applied potential, kinetics of the redox couple, and so forth. Therefore, the distribution of current pathways depends also on these variables. In the DSSC, the dark current will take the distributed path of least overall resistance (Sections III. A-III.C), meaning it will flow primarily through solution [50] under the expected conditions of Rct < / 2- The dark current is thus mainly a measure of reaction (5) in this potential range, even though reaction (4) is expected to be the dominant recombination... 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

In Table 1 only half-reactions involving the organic pollutants are indicated, and the species that act as a sink or source of electrons (i.e., the oxidants or i cdoctants, respectively) are not specified. Unfortunately, in environmental. vs(ems, in contrast to the reactions involving nucleophiles, to date, it has not been possible to identify which species react with an organic pollutant in a redox icaclion. Therefore, i( is often not possible to assess exact reaction pathways and In derive kinetic data that can be generalized. Consequently, with our present... 

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