A regenerative second-order reaction following electron transfer at an electrode surface produces a change in the apparent number of electrons transferred in the overall reaction. The reaction is usually written as the disproportionation of an electrode product A ⇄e- B (1-1) 2B →k A + D (1-2) This mechanism, however, leads to an interesting paradox since it implies that the reaction B ⇄e- D (1-3) proceeds more easily than reaction 1-1. Why, then, does not the overall reaction proceed as A ⇄2e- D (1-4) and how is it possible to observe second-order behavior? There are several solutions to this paradox ranging from the obvious expedient of declaring reaction 1-3 to be kinetically hindered, to some more complicated mechanisms involving additional kinetic steps. In this paper three alternatives to the disproportionation mechanism are presented and analyzed : self-dimerization of the electrode product and the subsequent electrochemical and chemical reaction of the dimer; dimerization of the electrode product with the parent and the subsequent electrochemical and chemical reaction of the dimer; and an equilibrium reaction of the electrode product preceding the second-order chemical reaction. These alternatives can exhibit behavior quite similar to the disproportionation mechanism when studied by the chronoamperometric technique.
