Carboxylic acid amide enols: keto-enol/enolate interconversion of N-methylindoline-2-one and its 2-thione and 2-selone analogs in aqueous solution

Carbon-acid ionization constants, QaK (concentration quotient at ionic strength = 0.10 M), were determined by spectrophotometric titration in aqueous solution for N-methylindoline-2-one, pQaK = 15.70, N-methylindoline-2-thione, pQaK = 8.93, and N-methylindoline-2-selone, pQaK = 7.25. Rate profiles were also constructed for the thione and selone. These were interpreted, with support from the form of acid-base catalysis as well as solvent and substrate isotope effects, as representing keto - enol/enolate ion interconversion. That led to the enol acidity constant pQaE = 4.05 and the keto-enol equilibrium constant pKE = 4.88 for the thione and estimates of the limits on these quantities, pQaE< 3 and pKE > 4, for the selone.Key words: lactam, thiolactam, selenolactam, keto-enol equilibria, carbon-acid ionization constants.