Ultrasonic Effects on Electroorganic Processes (19) Cathodic Reduction and Adsorption of p-Methylbenzaldehyde on a Liquid Mercury Electrode

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Ultrasonic effects on cathodic reduction and adsorption of p-methylbenzaldehyde in an acidic solution were investigated using a liquid mercury electrode. The reductions were also carried out on solid electrodes such as lead, amalgamated lead and cadmium under ultrasonic irradiation in comparison with the mercury electrode. Current efficiency for the galvanostatic electrolysis, leading to formation of the corresponding hydrodimeric (meso/dl-1, 2-di-p-methylphenyl-1, 2-dihydroxyethane; HD) and hydromonomeric (p-methylbenzyl alcohol; HM) products, was influenced by outlet power of ultrasounds at all the electrodes. In addition, product selectivities for HD/HM and meso/dl-HD were also increased and decreased, respectively, under the irradiation. However, these ultrasonic effects appeared quite different between the liquid and solid electrodes. A steady-state voltammogram of p-methylbenzaldehyde exhibited an unusual prewave with a maximum at −0.8 V vs. SCE on the liquid mercury electrode, while such a prewave could not be observed on the solid electrodes. The prewave disappeared under the irradiation as well as in the presence of a surfactant such as gelatin. The preparative scale electrolysis could be also potentiostatically carried out at −0.7 V vs. SCE in the prewave potential range in silence to give HD predominantly, while the electrolysis could not be carried out under the irradiation at such less negative potential. An electrocapillary curve in the solution of p-methylbenzaldehyde on the mercury electrode was drastically deformed from a parabola in a potential range of 0.2-0.9 V vs. SCE where the prewave was voltammetrically observed. These results suggested that the prewave might be as due to a self-catalytic reduction of p-methylbenzaldehyde adsorbed specifically on the mercury electrode.