Electrodimerization: VII. Electrode and solution electron transfers in the radical-substrate coupling mechanism. discriminative criteria from the other mechanisms in voltammetric studies (Linear sweep, rotating disc, polarography)
✍ Scribed by C.P. Andrieux; L. Nadjo; J.M. Savéant
- Publisher
- Elsevier Science
- Year
- 1973
- Weight
- 842 KB
- Volume
- 42
- Category
- Article
- ISSN
- 0022-0728
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✦ Synopsis
The formal kinetics of electrodimerization have been analyzed in a previous paper of this series 1, with particular emphasis on diagnostic criteria to be used in linear sweep voltammetry (LSV) and classical polarography (CP) for discrimination between various possible mechanisms.
It was shown that three kinds of mechanisms must be considered: (i) the radical-radical coupling mechanism ("DIM 1"):
A++e -~:~A" 2A'--*A-A (ii) the radical-substrate coupling mechanism ("DIM2") :
A++e -~=~A" A++A'---~'A-A + 'A-A++e -~=~A-A (iii) the ion-substrate coupling mechanism ("DIMY') which initially involves a two-electron uptake from the electrode :
The formalism used here, as in the previous study 1, deals with a positively charged substrate. This is purely conventional and the reasoning is strictly the same for, for example, a neutral substrate that gives rise to negatively charged monomer radical and dimeric species. This formalism has been used in other papers of the same series 2'3. Two situations have been considered giving rise either to a two ~electron wave system or to a one-electron wave system in each case. Only the last will be considered here since it is the more likely to occur in practice.
In the analysis of these various reaction schemes, and particularly for the "DIM2" mechanism, it was emphasized that the role of electron transfer reactions in solution should be considered as well as electron transfers at the electrode ~. Indeed, the reaction sequence written above for the "DIM2" mechanism is an e.c.e.-type scheme since the second electron transfer step is assumed to occur at the electrode surface. Since a one-electron wave is observed it is required that the dimer radical • A-A ÷ be more reducible than the substrate itself.
Under these conditions, the electron transfer reaction in solution:
A-A + +A" ~=~A + +A-A ,.