Rate and equilibrium studies of the reaction of oxyanions with 2-phenyloxazol-5(4H)-one

Abstract

Equilibrium constants for the reaction of phenoxide ions with 2-phenyloxazol-5(4H)-one at 25 °C and 1 M ionic strength obey a Brønsted relationship (log k_OH/k_ArO= log K′=–1.73pK^ArOH– 15.81) and are not subject to steric effects from ortho-substituents. Both forward and reverse rate parameters exhibit steric effects, and the Brønsted equations for meta- and para-substituted species are log k_OH=–0.81 pK^ArOH+ 9.75, and log k_ArO= 0.95pK^ArOH– 6.40. There is no break in the Brønsted line in the region of pK^ArOH 5–11, consistent with a single transition-state. An upward deviation exists for oxyanions with low basicity (pK^XOH < 5); one of these oxyanions, betaine, has a solvent deuterium oxide isotope effect for its reaction with the oxazolone greater than 2, consistent with a general base mechanism for attack for these species. The results for nucleophilic attack of phenolate anions are in agreement with a concerted displacement at the carbonyl group.