Solvent effects versus concentration effects in determining rates of base-catalyzed keto-enol tautomerization

Solvent effects of homogeneous media ( such as solvent - water mixtures) on chemical reactivity may be interpreted as due to solvent polarity and/or molecular structure of solvent molecules. In microheterogeneous media ( such as aqueous micellar solutions), solvent effects on reaction rates must include concentration effects, in addition to changes in the solvent polarity of the micelle interface where the reaction is assumed to occur. In this work, we measured the rates of keto-enol tautomerization of the 2-acetylcyclohexanone ( ACHE) and 2-acetyl-1-tetralone (ATLO) systems in dimethylsulfoxide ( DMSO) - water mixtures and in aqueous micellar solutions with both anionic and cationic surfactants and in the presence of buffers. The results appear as an ideal framework to understand the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent polarity or even concentration effects.