Sonochemistry of Alcohol-Water Mixtures: Spin-Trapping Evidence for Thermal Decomposition and Isotope-Exchange Reactions

Article abstract of DOI:10.1021/j100350a029

The sonochemistry of argon-saturated water-alcohol mixtures has been studied by ESR and spin trapping with 3,5-dibromo-4-nitrosobenzenesulfonate.Free-radical intermediates induced by 50-kHz ultrasound in aqueous solutions of ethanol, 1-propanol, 2-propanol, and 2-methyl-2-propanol were identified.Spin adducts typical of thermal decomposition of the alcohols and of H- and OH-induced abstraction reactions were observed.In the sonolysis of mixed-isotope systems of the type ROD-D2O and CH3CD2OH-H2O, isotopically mixed radicals were spin trapped, indicating the occurrence of multiple radical recombination reactions in the gas phase of the collapsing gas bubbles.The spin adduct yields of methyl radicals and of CH2CH2OH radicals obtained by sonolysis of water-ethanol mixtures were studied over a wide range of solvent composition, and two maxima were observed for both species.The first maximum occurs below 5 M ethanol and can be explained in terms of changes in spin-trapping efficiency due to competition between radical-spin trap and radical-ethanol reactions.The second maximum occurs at 10 M ethanol and is due to changes in acoustic cavitation.The effect of temperature on the sonochemical yield of 10percent ethanol was studied and the yield was found to decrease with increasing temperature in the range 25-50 deg C, indicating the predominant effect of vapor pressure of the bulk solvent on sonochemical yields.