Sonochemistry of Homogeneous Ionic Reactions

Besides the well-known role of ultrasound in initiation and acceleration of radical reactions, this low-energy acoustic field has recently become a valuable tool for physico-chemical investigations of solvation phenomena in homogeneous ionic reactions in aqueous and aqueous-organic solutions. In this mini-review we summarize these experimental data, showing that the application of classical quantitative methods, such as reaction kinetics and correlation analysis, if combined with sonication of the reaction medium, can reveal subtle hydrophobic interactions that remain hidden in the conventional experiments. Correlation of kinetic sonication effects with substrate hydrophobicity has revealed that independent of the reaction mechanism the decrease in reaction rate with the increasing content of different hydrophobic co-solvents can be explained by the reaction ground-state stabilization, which is largely of hydrophobic origin. When ultrasound suppresses this hydrophobic stabilization of reagents, it accelerates the reaction. On the contrary, if sonication perturbs the stabilization of encounter complexes between reagents, it hinders the reaction. Many of the conclusions reviewed in this article make a contribution to the chemistry of solutions and physical organic chemistry.