Kinetics and mechanism of ionization of the carbon acids 4'-substituted 2-phenyl-1,3-indandiones.

The ionization kinetics of 1.3-diketone carbon acids are slow relative to those of classical acids and bases. The ionization kinetics of three 4'-substituted 2-phenyl-1,3-indandiones, 4'-chloro-, 4'-methoxy-, and 2-phenyl-1,3-indandione itself, were studied at 25 degrees and ionic strength 0.1 using stopped-flow spectrometry and a pH jump technique. A log k'obs-pH profile for the approach to the ionization equilibrium was consistent with a reaction scheme postulated earlier for the ionization of another carbon acid, phenylbutazone. The percent enol versus diketo form of the acids and the pKaenol and pKadiketo were calculated from the kinetic data. Hammett plots of the various kinetic and equilibrium constants supported a mechanism for acid deprotonation consistent with proton abstraction being the predominant process when very weak bases such as water were the proton acceptors. Desolvation effects and the work required to get the two reacting molecules together in the correct configurations predominated when the proton abstraction was by stronger proton acceptors.