Chemical Kinetics of the Microwave Effect on the Base Hydrolysis Reaction Rate of Benzyl Isobutyrate
Authors: Baba Ryunosuke , Ohuchi Shokichi 
Published: August 2024

Many experimental results regarding Arrhenius plot on various chemical reactions under microwave irradiation have been reported. According to these results, it can be confirmed in a wide variety of researching fields on chemistry that gradient and/or intercept of Arrhenius plot are changed depending on presence or absence of microwave irradiation. From this phenomenological universality on changes in gradient and/or intercept and invariance of linearity of Arrhenius plot under microwave irradiation, it can be expected that these changes in Arrhenius plot are appropriate as quantitative definition of chemical reaction accelerating effects under microwave irradiation. In experiment in this study, in order to quantitatively evaluate chemical reaction accelerating effects as microwave-specific non-thermal effect, base hydrolysis reactions of ester, benzyl isobutyrate, as a common organic chemical reaction were performed under isothermal condition for each reaction temperature (40, 50, 60°C) in both presence and absence of TE10-mode continuous microwave irradiation condition (2.45 GHz, 50 W). By controlling reaction system temperatures by thermal transmission with microwave non-absorbing refrigerant, kerosene, while flowing in outer tube of double tube glass test tube, the reaction temperatures were kept at each target reaction temperature throughout the reactions with sufficiently high precision whether presence or absence of microwave irradiation. By using fiber optic thermometer for monitoring bulk temperatures of each reaction system and by pre-heating each reaction system before adding reaction substrate to initiate reaction, the discussions about microwave thermal effects in reaction progressing stage and reaction system temperature rising stage were excluded accurately. The quantitative evaluation of chemical reaction accelerating effects as microwave-specific non-thermal effect on the performed hydrolysis reaction was conducted by using extended Arrhenius equation proposed based on the phenomenological changes in Arrhenius plot commonly confirmed in various types of reactions. As the result of this experiment, two types of correction coefficient of formula of Arrhenius plot for applying microwave irradiating reaction condition, “enthalpic effect coefficient” CH and “entropic effect coefficient” CS, were obtained as the chemical reaction accelerating effects from the analysis on gradient and intercept of experimentally drawn Arrhenius plot. The linearities of Arrhenius plots were sufficiently appropriate with or without microwave irradiation. In addition to confirming the chemical reaction accelerating effects under microwave irradiation on the performed hydrolysis reaction, from comparing between the orders of experimentally obtained values of CH and CS, it was suggested as one of the mechanisms of microwave-assisted chemistry that microwaves affect not on activation enthalpy ΔH°‡ and activation entropy ΔS°‡ separately, but on both values in the same mechanism caused by affecting on activation Gibbs free energy ΔG°‡ with microwaves. According to this suggestion, another definition of chemical reaction accelerating effect under microwave irradiation, “work effect coefficient” CG, was also proposed based on transition state theory in addition to CH and CS. From the results in this study as demonstration of leveraging extended Arrhenius equation, it is expected that the methodology in this study using extended Arrhenius equation including CH and CS is able to contribute to develop knowledge in the fields on microwave-assisted chemistry.