Ab initio and electrochemical studies on the reductive bond dissociation in haloethanols

Electrochemical behavior of two water-soluble molecules, namely, 2-bromoethanol (Br-EtOH) and 2-iodoethanol (I-EtOH), on glassy carbon and on group IB metal electrodes is investigated. Because the carbon-halogen (C-X) bond cleavage in these haloethanols is a totally irreversible process, the corresponding redox potentials were determined from the ab initio calculations and/or from the known C-X bond dissociation energy in these molecules. The gas phase Gibbs energies of formation for the reactants and the reaction products were obtained at the MP2/6-31G** level and were further corrected for the hydration of molecules using the Polarization continuum model (PCM) extended for the calculations of polarization, repulsion and cavitation contributions to the total solvation energy. The overvoltage for the I-EtOH reduction on Ag(111) and polycrystalline Ag is shown to be approximately 0.4 V lower than on glassy carbon electrode. The electroreduction of I-EtOH at Cu(111) and Cu(100) electrodes takes place at potentials only slightly more negative than that on silver. In all cases, the observed overvoltages are much lower than those predicted by the current dissociative electron transfer model. Gas chromatographic analyses revealed ethylene being the major reaction product, with only small amounts (4-10%) of ethanol formed during the bulk electrolysis runs resulting practically in a complete destruction of iodoethanol.