Molecular Size Effects in the Kinetics of the Electroreduction of Nitroalkanes on Mercury

The reduction of nitroalkanes at the dropping mercury electrode in neutral solutions (pH 5.4–7.4) was studied. Careful measurements at the foot of the polarographic wave allow accurate determination of the Tafel slopes to within ± 1 mV, thus the effect of molecular size on the Tafel slope could be determined. The analysis of the results indicates that the adsorbed intermediate, which is a charged species, is situated about halfway between the metal surface and the outer Helmholtz plane. It is also shown that the intermediate is adsorbed with the functional nitro group lying flat on the surface. Only the carbon atoms in the position α and β to the nitrogen atom seem to occupy space on the surface. The importance of employing several members of a homologous series having a common functional group to evaluate the mechanism of electrode reactions is emphasized and the importance of employing the combined isotherm in such analysis is pointed out.