Reduction of 2-cyclohexen-1-one in aqueous media on a mercury electrode

Abstract

A more comprehensive study than those previously reported in the literature has been carried out on the electrochemical reduction of 2-cyclohexen-1-one in aqueous solutions over a wide pH range. The influence of the pH, reactant concentration and drop time on the different polarographic and kinetic parameters, such as the limiting current, half-wave potentials, Tafel slopes and reaction orders with respect to 2-cyclohexen-1-one and the H + ion, is studied for the two electroreduction processes involved. A comparison between experimental DP curves and theoretical profiles corresponding to different mechanisms is also included. The experimental results clearly indicate that the process corresponding to the first wave (pH < 8) involves the protonation of 2-cyclohexen-1-one at the carbonyl group, followed by an electron transfer to yield a neutral radical which dimerizes in the reaction layer. This dimerization step is the rds at potentials corresponding to the foot of the wave, whereas diffusion at 3 < pH < 5 and the previous protonation at pH > 7, govern the process at potentials corresponding to the top of the wave. Regarding the second wave (pH > 5), which is due to the reduction of the unprotonated form of 2-cyclohexen-1-one, a change of mechanism can be observed as the pH increases. This change is due to the fact that the dimerization step can take place via either neutral radical—neutral radical (RH π + RH π) or radical anion—radical anion (R π− + R π−) coupling, depending on both the pH and the concentration of proton donors in the solution.