LOW-TEMPERATURE STUDIES OF ELECTROCHEMICAL KINETICS .3. USE OF CONCENTRATION STUDIES IN DETERMINING THE RATES, MECHANISMS, AND APPARENT REACTION ORDERS OF ELECTROHYDRODIMERIZATION REACTIONS

The homogeneous reaction kinetics of products formed by the reduction of methyl (MC) and ethyl (EC) cinnamate in tetra-n-butyl ammonium iodide-dimethyl formamide have been investigated at several temperatures. These electrohydrodimerization (EHD) reactions have been studied with double potential-step chronocoulometry (DPCC). Cyclic voltammetry experiments were used to determine the step potential necessary to obtain diffusion-limiting conditions for the variable temperature DPCC experiments. A digital simulation program has been written that treats EHD as a preequilibrium of radical ions followed by a first order rate-determining step; this allows one to obtain theoretical apparent reaction orders between 1 and 2. Experimental variation of the bulk concentration of these compounds at several temperatures has yielded results that are comparable to the theoretical results obtained by digital simulation. This allows one to estimate the equilibrium constant for the dimerization preequilibrium and the (pseudo) first order rate constant associated with protonation or ring closure. It has been found that MC exhibits behavior similar to that of EC and is also compatible with the proposed theoretical model. At 25°C the rate constants for MC and EC were found to be 9.2 and 3.7 sec-1, respectively; energies of activation for the two compounds were 5.8 and 5 9 kcal/mole. At the same temperature the equilibrium constants for MC and EC were 53.1 and 109.0M-1, respectively, and exhibited little change with temperature variation. Apparent reaction orders ranged between 1.6 and 1.7. © 1979, The Electrochemical Society, Inc. All rights reserved.