Ionization kinetics of the carbon acid phenindione

The ionization kinetics of carbon acids are slow relative to those of classical acids and bases. Phenindione (2-phenyl-1,3-indandione) is a 1,3-diketone carbon acid of macroscopic experimental pKa 4.09 at 25 degrees and ionic strength 0.1. The ionization kinetics of phenindione were determined at an ionic strength of 0.1 and 25 degrees using stopped-flow spectrophotometry and a pH jump technique. A log k'obs-pH profile for the approach to the ionization equilibrium was determined, and a mechanism consistent with the profile was postulated. The percent enol versus diketo form of phenindione and the pKaenol and pKadiketo were calculated from the kinetic data. Phenindione acid deprotonation kinetics by various oxygen and nitrogen bases suggested that, with bases of pKa 4.7--8.5 and in the pH 5--8.5 range, the acidic phenindione proton as approximately 45% transferred in the rate-determining transition state.