Proton-Transfer Reactions of Dibasic Acids in Aqueous Solution: 3-Hydroxypyridine and Anthranilic Acid

Reaction enthalpies and reaction volumes of proton-transfer reactions in aqueous solutions of 3-hydroxypyridine have been estimated by measuring UV-visible absorption spectra at different temperatures, pressures, and pH values. Using these results together with published thermodynamic and kinetic data, we have simulated the relaxation spectrum of the proton-transfer reactions of 3-hydroxypyridine in aqueous solution. The first process identified in our normal mode analysis is the relaxation of the well-known tautomeric equilibrium between the neutral form and the zwitterion, AN = A,. The second relaxation process is the overall reaction A+ + A= XAN + (2 x)Az, with x being roughly equal to unity. The amplitude of this relaxation is significantly smaller than that of the first. The third relaxation process consists of two branches. The first branch is the overall reaction A+ = XAN + (1 x)A2 + H+, x 0.5, at pH << 7, and the second branch is the overall reaction XAN + (1 x)AZ + OH= A+ H20, x 0.5, at pH >> 7. The fourth relaxation process is more complicated than the others. It consists mainly of hydrolysis reactions at pH << 7 and protolysis reactions at pH >> 7. This relaxation process can be neglected because of its small amplitude. In the second part of this work, we have measured the ultrasound absorption of aqueous solutions of anthranilic acid, and using the information obtained from the normal mode analysis of 3-hydroxypyridine, we present evidence for the intramolecular proton-transfer reaction AN + A,. The sum of forward rate constant kz3 and reverse rate constant k32 is about 9.3 X lo6 s-'. With the literature value of 0.2 for the equilibrium constant, the rate constants are kz3 1.6 X lo6 s-' and k32 7.8 X lo6 s-'. In an earlier study, White and Slutsky had found no evidence for this reaction. However, because they had only a few data, they could probably not clearly distinguish between the different contributions to the measured relaxation process