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, Q(a)(K) (concentration quotient at ionic strength = 0.10 M), were determined by spectrophotometric titration in aque ous solution for N-methylindoline-2-one, pQ(a)(K) = 15.70, N-methylindoline -2-thione, pQ(a)(K) = 8.93, and N-methylindoline-2-selone, pQ(a)(K) = 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 s olvent and substrate isotope effects, as representing keto - enol/enolate i on interconversion. That led to the enol acidity constant pQ(a)(E) = 4.05 a nd the keto-enol equilibrium constant pK(E) = 4.88 for the thione and estim ates of the limits on these quantities, pQ(a)(E) < 3 and pK(E) > 4, for the selone.