Current dips in polarography and cyclic voltammetry associated with electrochemical inducement of chemical reactions: Aromatic nucleophilic substitution

Abstract

In electrochemically induced chemical reactions, such as aromatic nucleophilic substitution, current dips are frequently observed in cases where the product wave is negative to the reactant wave. The dips then appear following the reactant wave at the foot of the product wave. Such phenomena have been found for electrochemically induced aromatic substitutions in organic solvents or in liquid ammonia. They can be observed both in cyclic voltammetry and classical polarography. A mechanistic and kinetic model is presented which accounts for the formation of these current dips in the context of the two techniques. The effect of the experimental parameters on the shape, magnitude and location of the dip is discussed. It is shown that the main cause for the appearance of the current dips is the occurrence of an autocatalytic process triggered by a solution electron transfer between the reduced form of the product and the reactant. It does not involve surface phenomena, although the current—potential curves resemble those found in the case of self-inhibition deriving from reactant adsorption or passivation phenomena. The similarity in the experimental manifestation of both types of phenomena also appears through the observation of hysteresis in the potential—current curves obtained in galvanostatic conditions for electrochemically induced reactions.