Two -electron reactions in organic electrochemistry

The electrochemical oxidation or reduction of many organic and organometallic compounds proceeds by two or more individual electron-transfer reactions. In chapter 1, various aspects of such reactions are reviewed. These include the factors that govern the difference between the formal potentials for the two steps (Eo′1 − Eo′ 2), causes of potential inversion (Eo′1 − Eo′2 < 0 for reductions) and mass transfer and kinetic effects associated with the disproportionation/comproportionation reaction among the three species involved in the two-step reaction.;In chapter 2, steady-state microelectrode voltammetry has been used to obtain reversible half-wave potentials for the two reduction waves of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the oxidation wave of ferrocene, in acetonitrile with concentrations of tetramethylammonium hexafluorophosphate electrolyte ranging from 1 to 70 mM. The dependence of the first half-wave potential of TCNQ, referred to the half-wave potential of ferrocene, on ionic strength was adequately accounted for by changes in activity coefficients of ferrocenium ions and TCNQ radical anions as predicted by the Debye-Hückel equation. It is concluded that there is no significant ion pairing of either ferrocenium ions with hexafluorophosphate from the electrolyte or TCNQ radical anions with tetramethylammonium ions of the electrolyte.;The effect of comproportionation on voltammetric peak shape when the second electron transfer is irreversible is examined in chapter 3. Normally, this comproportionation reaction has little or no effect in voltammetry. In steady-state voltammetry the normally symmetric, sigmoid-shaped second wave is predicted to rise more sharply near its base than is observed in the absence of comproportionation and, in the limit of a very fast comproportionation reaction, an “onset potential” develops at which the current at the second wave increases abruptly from the limiting current of the first plateau.;In chapter 4, the reduction mechanism for trans-2,3-dinitro-2-butene was determined. A variety of solvents and electrode types were used. Quantitative results could only be obtained in a mixture of acetonitrile (83%) and water (17%) on a mercury electrode. The mechanism was found to be a two-electron transfer, which occurred with potential inversion.;The electrochemical reduction of eight quinones (9,10-anthraquinone, duroquinone, 2,6-di-tert-butyl-1,4-benzoquinone, 2,6-dimethoxy-1,4-benzoquinone, 9,10-phenanthrenequinone, tetrachloro-1,2-benzoquinone, tetrabromo-1,2-benzoquinone and 3,5-di-tert-butyl-1,2-benzoquinone) has been studied in acetonitrile and will be examined in chapter 5. In every case it was found that cyclic voltammograms differed in significant ways from those expected for simple stepwise reduction of the quinone to its radical anion and dianion. The various types of deviations for the eight quinones have been cataloged and some speculation is offered concerning their origins. In chapter 6, the mechanism for reduction of 3,5-di-tert-butyl-1,2-benzoquinone is determined and found to involve a dissociative electron transfer mechanism, the reduction of a hydrogen-bonded complex of the radical anion with water or other hydrogen-bond donors.