The Kinetics of the Electroreduction of Acetone

Abstract

The electrolytic reduction of acetone in alkaline aqueous solutions at a mercury surface has been studied with the object of determining the kinetics of the formation of isopropyl alcohol and pinacol. The reduction rate has been studied at various acetone concentrations, in varying concentrations of sodium hydroxide, in solutions of sodium sulfate, sodium chloride, barium hydroxide, and potassium hydroxide. During electrolysis the rates of formation of isopropyl alcohol and pinacol were followed continuously. It is found that at low acetone concentrations the rate of formation of isopropyl alcohol is proportional to the acetone concentration, while the rate of formation of pinacol is proportional to the square of the acetone concentration. At somewhat higher acetone concentrations the pinacol rate becomes linear in acetone concentration, and near saturation the isopropyl alcohol rate becomes practically independent of acetone concentration. At constant current density, increasing the sodium hydroxide concentration decreases the yield of isopropyl alcohol and increases the yield of pinacol (except at high concentrations of acetone and sodium hydroxide). Substitution of chloride or sulfate for hydroxide causes an increase in the alcohol rate but lowers the pinacol rate. Substitution of barium for sodium stops the reduction completely. Substitution of potassium for sodium increases both yields. Increasing the current density is accompanied by a decrease in the fraction of the current which goes into isopropyl alcohol formation and pinacol formation. It is shown that these experimental results can be explained by a modification of the mechanism proposed by Müller. The acetone is first adsorbed on the mercury with the formation of a covalent bond. The product of the adsorption may dissociate from the surface as a free radical, in which case pinacol is the final product, or it may first add on a proton and then dissociate from the surface as an ion which adds a second proton to become isopropyl alcohol.