The electrohydrodimerization of maleimide in buffered aqueous solutions : a kinetic and structural analysis

The one electron reduction of maleimide in neutral aqueous solutions was found to produce dimers exclusively. Characterization of the product by gravimetric microanalysis, HPLC, NMR and IR gave as the sole product 1,2,3,4 butanetetracarboxylic 1,2:3,4 diimide. Polarographic limiting currents were found not to be diffusion controlled but both chemically and diffusion controlled depending on the concentration of initial maleimide. Thus only the apparent number of electrons transferred could be calculated and these values were found to vary from unity at concentrations above 1 millimolar to a value of two at concentrations 0.05 millimolar and below. This concentration dependence could not be attributed to association in solution through hydrogen bonding of the reactant since UV analysis of these solutions compared favourably with those published in the literature for the monomer. Adsorption of maleimide in the phosphate buffered solutions was not detected by double layer impedance measurements conducted with and without maleimide present. Controlled potential coulometry of maleimide from 0.1 to 10. millimolar gave an average value of 0.9 for the number of electrons transferred at the potential of the first polarographic wave.Analysis of the cyclic voltammetric curves at pH 7.0 for the kinetic pathway applicable indicated a radical-radical coupling with protonation of the dimer anion as the rate controlling step at concentrations less than 0.1 millimolar while at concentrations greater than 10. millimolar, radical-radical coupling is the rate determining step.This dimerization mechanism was inferred over the entire concentration range by the 40 millivolt change in potential per decade change in current (Tafel slope), the expected variation in the peak current with the voltage sweep rate, and the shifting in peak potential with the logarithm of concentration and voltage sweep rate. This concentration dependence was further indicated by the cyclic voltammetric pattern at voltage sweep rates of 200 volts/second and above. At low concentrations (<.1 mM) oxidation currents were seen in the voltammograms but at the higher concentrations (>10. mM) oxidation currents could not be detected.