Sonoelectrosynthesis Of Benzaldehyde And Its Derivatives With Mid-valence Manganese As Medias

As an important species of organic compound, benzaldehyde and its derivatives were widely used in synthesis of many kinds of chemicals, such as pharmaceuticals, spiceries, cosmetics, dyes, pesticide, etc. Studying theoretically and practically on the methods for its preparation has been all along a significant project for the chemist, since it has both good economic benefits and high academic values.In this paper, benzaldehyde and its derivatives were prepared by sonoelectrosynthesis method, of which the course and the mechanisms were studied at the same time. In the research, toluene and xylene were employed as raw materials, Mn(III) and particle MnO₂ obtained from sonoelectrolyzed Mn(II) were employed as oxidants, and they were applied to prepare benzaldehyde and its derivatives under the ultrasonic irradiation. Meanwhile, considering the more and more outstanding position of ultrasonic technology in nano-materials, manganese dioxide particle of nano-size were attempt to be prepared as well with sonochemical method in the experiment.When ultrasound was brought into the course of electrooxidating Mn(II) to Mn(III), it could continuously swash and clean the electrode surface, cleared away the air bubbles around the electrode and maintained its activity. As a result, mass transfer rate was increased, while oxidative potential was decreased, and current efficiency was promoted. Therefore, it made the electrolytic reaction time less and the concentration of sulfuric acid lower than those by mechanical stirring. All mentioned above indicated a special function of ultrasound. Moreover, as ultrasound was employed in the heterogeneous reaction of preparation for benzaldehyde with Mn(III), high yields could be obtained in a short time, and the oxidant solution could be reproduced and utilized repeatedly by electrolysis with little changes of the effect that less than 4% and 2% decrease in the current efficiency and benzaldehyde's yield respectively. While the method was applied to prepare p-,m-,o-tolualdehyde by oxidizing corresponding xylene, the effect of ultrasound was more remarkable. Concentration of sulfuric acid needed was reduced, and product's yield was apparently improved compared with stirring. The yield of p-tolualdehyde, m-tolualdehyde, and o-tolualdehyde was increased by 4.3%, 19.0% and 20.7%, respectively.In the experiment, preparation conditions of particle MnO₂ by sonoelectrooxidation method were optimized, and effect mechanisms of different factors such as electrode materials, ultrasonic frequency, ultrasonic function power, current density, electrolytic temperature, concentration of manganese sulfate and concentration of sulfuric acid were discussed. Particle MnO₂ close to nano-size was produced, and its characteristic was determined by laser diameter analyzer, IR and XRD. In the experiment it was found that, under the ultrasonic irradiation, particle size of MnO₂ produced by grid Pt electrode was much smaller than that produced by PbO₂-Pb electrode. However, either of them was much smaller than that reagent MnO₂, and with the ultrasonic frequency increasing, particle size of the product minished a little. Meanwhile, the test results showed no matter which electrode was selected,γ-MnO₂ crystal was the major product in the end. When used in oxidizing toluene to prepare benzaldehyde and compared with analytical reagent MnO₂, it behaved with a higher oxidative activity and a higher yield of benzaldehyde.Through the experiment, it was also found that, similar results were found either Mn(III) or MnO₂ was employed for oxidizing xylene to prepare tolualdehyde. That was, paraxylene was most likely to be oxidized, then the metaxylene, while the orthaxylene was the last. The sequence of the product's yield was p-tolualdehyde > m-tolualdehyde > o-tolualdehyde. Moreover, yields of the target products were apparently promoted as ultrasound was imposed on.