Ionization of Acetylacetone in Me2SO—Water Mixtures. Reactivity—Selectivity Principle or Principle of Imperfect Synchronization?

Rate constants for the reversible deprotonation of acetylacetone were measured in carboxylate and amine buffers in water and in 50%, 90% and 95% Me₂SO at 20°C. The Brønsted plot for the carboxylate ions is curved in the Me₂SO—water mixtures, but straight in water. The curvature is in the direction predicted by the Reactivity—Selectivity Principle (RSP). However, the Brønsted plot for the reaction with primary amines is straight in all solvents. This suggests that the curvature observerd with the carboxylate ions is caused by loss of solvation of the base; this loss of solvation is ahead of bond formation in the transition state rather than being a manifestation of the RSP. (Note that all Brønsted plots are based on pK_a values measured in the respective solvents.) The intrinsic rate constant (k₀) for proton transfer increases with the addition of Me₂SO, and more so with the carboxylate buffers than with the amines. This increase in k₀ is attributed to delayed solvation of the developing enolate ion in the transition state; with the carboxylate buffers, an additional factor is the early loss of solvation of the base. The various solvation effects observed in this study can all be understood in the context of the Principle of Imperfect Synchronization (PIS).