14. 有機ソノケミストリーにおける溶媒の役割

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In 1991 we reported that the irradiation of ultrasound switched the course of the reaction of lead tetraacetate (LTA) with styrene in acetic acid from ionic one to radical. We have studied the detailed mechanism of the sonochemical excitation of the reaction. First, we thought that ultrasound accelerates the decomposition of LTA to produce methyl radical at the interface of the bubble. However, we found that the decomposition of LTA itself was not accelerated by ultrasound in the absence of styrene. Then, we observed a good linear free energy relationship (LFER) between the radical reactivity of 4-substituted styrenes with LTA and the vapor pressure of the styrenes, which indicates that the real activated species in the cavitation process must be volatile styrenes. We then observed a similar LFER for simple sonochemical excitation of substituted styrenes by quenching the radical species with pyrocatechol. However, there are still some ambiguities. First, direct activation of styrenes by sonication is unlikely to produce the radical cation species, but to yield benzene and acetylene by analogy with pyrolysis or shock wave decomposition. Second, the solvent acetic acid is much more abundant and volatile than styrenes under the experimental conditions used. The bubble atmosphere is mainly constituted by acetic acid. Thus, we studied the sonolysis of acetic acid itself. Sonolysis of acetic acid, pure or with added KOAc in the presence and absence of 1% water, was carried out with a thermostated glass reactor at 200 kHz at 25 ℃. Gaseous products evolved were followed by GC. Constant decomposition and evolution of gases were observed. Total volume of gases after 30 h sonication was about 2-3 mL from 30 mL of acetic acid,B Relative amounts of gases produced are in the following order: CO >> CH_4 > CO_2>H_2>>C_2H_6=C_2H_4. Although the detailed mechanism of the production of these gaseous products in sonolysis of acetic acid is still in speculation, it is evident that an oxidative species such as acetoxyl radical produced in this process must oxidize styrene to its radical cation in the gas phase of the bubble. The more important conclusion of this study is that most sonochemical reactions in organic solvents may originate in the sonolysis of solvent molecules as similarly as in aqueous sonochemistry.
日本ソノケミストリー学会の論文
2001-11-28

14. 有機ソノケミストリーにおける溶媒の役割

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