ISOTOPE RATE EFFECTS IN THE ENOLIZATION OF OXALACETIC ACID

Abstract

Acid-catalyzed enolization of oxalacetic acid in H2O and D2O gives rise to a constant isotope rate effect [Formula: see text] of 2.4 over the pH (pD) range of 5.5 to 7.5. Base-catalyzed enolization of oxalacetic acid in H2O and D2O gives rise to a constant isotope rate effect [Formula: see text] of 4.5 over the pH (pD) range of 7.0 to 8.0. The percentage of enol form of oxalacetic acid was calculated to be 15.3% at pH 8.0, using the absorption of the enol form in ether at 255 mμ and the absorption of the keto form at pH 0.5 in an aqueous system. The spectrophotometric measurement of the isotope rate effect involved in the enolization of oxalacetic acid gives direct evidence for the rate-determining step in both the acid- and the base-catalyzed reactions without subsequent reactions.