Oxidation of anthracene on platinum macro- and micro-electrodes: Sonoelectrochemical, cryoelectrochemical and sonocryoelectrochemical studies

Abstract

The electro- and sonoelectro-chemical oxidation of anthracene in acetonitrile has been explored at room and low temperature (−40 °C) at a platinum micro- and macro-electrode, respectively. From both methodologies, the effective number of electrons, n eff, in the oxidation process is evaluated at both room temperature and at the limit of the solvent–electrolyte freezing point. It is observed that n eff is close to 1 at low temperature while at room temperature it is higher, indicating sluggish follow up kinetics at lower temperature. The oxidative electrolysis of anthracene within an undivided cell, under various conditions is investigated with the product(s) obtained analysed and quantified by 1H NMR. At room temperature under both ‘silent’ and ‘sono’ conditions the electrochemical oxidation produces 9,10-anthraquinone and bianthrone while at low temperature with acoustic streaming, anthracene-9,10-diol is additionally produced. Electrolyses with intentionally added water and also with oxygen saturation is investigated and its influence on the reaction pathway reported. In particular, the addition of water (1.4 M) favours the formation of anthraquinone in a 3:1 ratio with bianthrone. Overall, the effect of lower temperature is to favour the formation of anthraquinone or its reduced form whilst the addition of insonation at low temperature is to substantially enhance the product yield per Coulomb of charge injected.