Chemins réactionnels en électrochimie organique : adsorption et protonation

Abstract
The electrochemical reduction or oxidation of organic compounds in aqueous medium is characterized by two particular features. a) Practically all the organic substances are adsorbable, to a lesser or to a larger extent. b) Fast protonation of the intermediates leads to apparent global n-electron reactions. The effects of the adsorption of the redox species are first analyzed. It is shown that for weak or intermediate adsorption (mobile film) the reaction appears as heterogeneous, with an apparent constant k_happad, whose order of magnitude is about 10⁴ to 10⁶ times larger than the "normal" heterogeneous rate constant. This is because reduction or oxidation takes place via a surface reaction, whose rate constant k_s is very large (of the order of 10⁹s^-1). Both k_happad and k_s are too large to be determined by existing electrochemical methods. The theory of square (cubic, bicubic, etc...) schemes with protonations at equilibrium shows that any of these reactions is equivalent to simple electrochemical reactions, whose apparent constants are much decreased, in comparison with the elementary rate constants. Application of this theory enabled us to determine k_s and k_happad for different compounds in the whole pH range ; several examples (4-nitropyridine and its N-oxide, ...) are presented. Our study confirms the order of magnitude indicated above for k_s and k_happad, and allows the detailed mechanism (sequence of addition of electron and protons) to be determined.