Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pK_a Values for Twenty Triazolium Salts in Aqueous Solution

Second-order rate constants have been determined for deuteroxide ion-catalyzed exchange of the C(3)-proton for deuterium, k(DO) (M-1 s(-1)), of a series of 20 triazolium salts in aqueous solution at 25 degrees C and ionic strength I = 1.0 (KCI). Evidence is presented that the rate constant for the reverse protonation of the triazol-3-ylidenes by solvent water is close to that for dielectric relaxation of solvent (10(11) s(-1)). These data enabled the calculation of carbon acid pK(a) values in the range 16.5-18.5 for the 20 triazolium salts. pD rate profiles for deuterium exchange of the triazolium salts reveal that protonation at nitrogen to give dicationic triazolium species occurs under acidic conditions, with estimates of pK(a)(N1) = -0.2 to 0.5.