Comparison of Heterogeneous and Homogeneous Electron-Transfer Rates for Some Nitroalkanes and Diketones

Article abstract of DOI:10.1021/j100185a026

Redox catalysis has been used to investigate the homogeneous electron-transfer reactions between the substrates (nitroalkane or diketone) and the reduced form of the catalyst, which was terephthalonitrile.The substrates were RNO2 (R=Me, Et, i-Pr, and t-Bu) and RC(O)C(O)R (R=Me, Et, and t-Bu).The experiments were conducted in acetonitrile solvent at 298 K with three different electrolytes, R4NClO4 (R=Et, n-Bu, and n-C7H15).The effect of added water and alcohols (methanol, ethanol, and s-butanol) was investigated.Formal potentials and standard heterogeneous electron-transfer rate constants, k_s, were also determined.It was found that increasing the size of the cation of the electrolyte resulted in a decrease in k_s but did not affect the rate of the homogeneous electron-transfer reactions.The result is interpreted as a decrease in electron-tunneling rate with increasing thickness of a compact layer of adsorbed cations, a double-layer effect that cannot influence the homogeneous reaction.Addition of the hydroxylic solutes, S, caused diminution of both k_s and the rate constant of homogeneous electron transfer from the anion radical of the substrate to the catalyst.This result is discussed in terms of two models, viz., complexation or adduct formation between the anion radical and S or increasing solvation energy of the anion radical.