Nuclear Magnetic Resonance Investigation of the Spontaneous Decarboxylation of 2-Oxalopropionic Acid. 2. Species in Solution

Article abstract of DOI:10.1021/ja00415a032

The kinetics of the spontaneous decarboxylation of 2-oxalopropionic acid (OPA) to the enolate intermediate of α-ketobutyric acid (AKBA) with subsequent ketonization, and β-deuteration via enolization, have been studied by NMR in aqueous solution at 31 deg C.The rate constants for the decarboxylation of the fully protonated, monoprotonated, and fully deprotonated species of OPA were found to be 1.67 * 10^-5 s^-1, 13.5 * 10^-5 s^-1, and 7.75 * 10^-5 s^-1, respectively.The rate constant for the ketonization of the intermediate was found to be 3.25 * 10^-4 s^-1 while the rate constant for the enolization of OPA was found to be 2.70 * 10^-4 s^-1.The ketonization and enolization processes exhibited specific acid catalysis and the second-order rate constants were found to be 1.60 * 10^-1 M^-1 s^-1 and 1.20 * 10^-1 M^-1 s^-1, respectively.The first pK_a of OPA, involving the carboxyl adjacent to the keto function, was found to be 1.75, while the second pK_a for the remaining carboxyl group was determined to be 4.18.In D2O the pK's were calculated as 2.38 and 4.50, respectively.Under the reaction conditions employed the hydrate species exists in appreciable concentrations at low pH while the concentration of the enol species was not significant.