Electrolysis in the presence of ultrasound: Cell geometries for the application of extreme rates of mass transfer in electrosynthesis

Two types of sonoelectrochemical reactors for preparative synthetic work are suggested and characterized by their performance in the reversible one electron reduction of cobalticenium cations in acetonitrile solution. The dominant effect of ultrasound is to strongly agitate the liquid phase and the corresponding parameters (mass transport coefficients) for the transport at the electrode surface are reported. An upper limit for ultrasound induced mass transport at macroelectrodes an order of magnitude beyond that achieved in conventional electrolysis is described. Further, for a synthetic scale reaction, the reductive ring opening of the α,β-epoxyketone isophorone oxide to yield the corresponding β-hydroxyketone, an improved current efficiency (3 F for a direct two electron reduction) and a clean conversion (yields up to 80% isolated) are demonstrated. The extremely high rates of mass transport achieved in the presence of ultrasound considerably enhance the performance and current efficiency in electrosynthesis by selectively increasing the rates of the mass transport controlled processes.