Dissociative electron transfer reaction

FIGURE 34.8 Free-energy surfaces for the dissociative electron transfer reaction (a) for the solvent polarization (b) along the coordinate r of the molecnlar chemical bond. corresponds to stable molecule in oxidized form. U" is the decay potential for the rednced foim. AFj and AF are the partial free energies of the transition determining mntnal arrangement of the two sets of the free-energy surfaces. 

Coming back to thermal homogeneous dissociative electron transfer reactions, the question arises whether the electron-donor molecule reacts as a single electron donor or as a nucleophile in an Sn2 reaction. We will review this long-debated question in Section 7, including the most recent developments. 

The thermodynamics of dissociative electron transfer reactions were first characterized by Hush12 from thermochemical data in the case of the electrochemical reactions... 

Recently2 it has been asserted that the very existence of dissociative electron transfer reactions is ruled out by application of the principle of microscopic reversibility. The line of argument was as follows. In the reaction of the cleaving substrate RX, say, with an electron donor D (the same argument could be developed for an oxidative cleavage triggered by an electron acceptor),... 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

The forward and reverse rate constants are thus equal at zero standard free energy. However, this will be difficult to check in practice, for both reactions are very slow, since a bond-breaking/bond-forming process endowed with a quite large internal reorganization is involved. The result is that dissociative electron transfer reactions are usually carried out with electron donors that have a standard potential largely negative to the dissociative standard potential. The reoxidation of the R, X- system is thus possible only with electron acceptors, D +, that are different from the D,+ produced in the reduction process (they are more powerful oxidants). There is no reason then that the oxidation mechanism be the reverse of the... 

The fact that the anion radical is an intermediate in this case falls in line with the observation that it is also an intermediate in the reduction of the same substrates by homogeneous or heterogeneous outer sphere electron donors and also that nitrobenzyl halides are quite easy to reduce (see Section 2, p. 66). In the other cases, the generation of the R radical has been assumed to proceed by halogen-atom transfer (158). It should, however, be noted that an outer sphere, dissociative electron-transfer reaction (163) would also... 

Figure 4.6. Homogeneous dissociative electron transfer reaction between aromatic radical-anions and (a) di-(4-cyanophenyl) disulphide, (b) diphenyl disulphide in dimethyl formamide. Data from ref [31J.

The development of a consistent theory for a dissociative electron transfer is a recent challenge in the field of theoretical electrocatalysis. Progress in this field of electrochemistry has involved the use of an harmonic Morse curves [25] instead of harmonic approximations. Applying the principles of the theory of the activated complex to adiabatic dissociative electron transfer reactions, the work of Saveant resulted in the following expressions [24] for the Gibbs energy of activation... 

As described in Section 4.1.2, electrophilic organic compounds are reducible at the electrode. Some reducible organic compounds are listed in Table 8.5 with the potentials of the first reduction step in dipolar aprotic solvents. As described in Ref. [47], organic compotinds undergo various complicated electrode reductions. Here, however, only simple but typical cases are considered they are reductions of the outer sphere type and the dissociative electron transfer reactions. 

Kuznetsov, A.M., German, E.D., Masliy, A.N. and Korshin, G.V. (2004) A density functional study of dissociative electron transfer reactions with partidpation of halogenated methanes. J. Elec-troanal. Chem. 573, 315-325. 

Another application of redox catalysis is the study of dissociative electron transfer reactions (Scheme 1). The resulting free radical R may undergo either of two reactions, coupling with the mediator radical anion (iii) or reduction to R (iv) [128] (Scheme 8). The coupling reaction is usually considered as unwanted since the mediator is irreversibly consumed in this step. The reaction is, however, synthetically useful [128],... 

Logarithmic analyses can be carried out on the neopolarogram in much the same way as with classical polarography (see Sec. V). Examples that illustrate the application of the convolution technique include reversible dimerization of radical cations [114], the study of dissociative electron transfer reactions [175,176], and investigations of the possible potential dependence of the transfer coefficient a [174,177]. 

Ignaczak A, Schmickler W. Theoretical study of a non-adiahatic dissociative electron transfer reaction. J Electroanal Chem 2003 554—5 201-9. 

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