A dielectric and spectrophotometric study of the tautomerization of 2-hydroxypyridine and 2-mercaptopyridine in water.

The basic ionization (p k 1 ) and acidic ionization (p k 2 ) constants and equilibrium constant ( K T ) of 2HPy and 2MPy were determined. The p k 1 (s) of their N - and X -methyl derivatives (X = O, S) were also determined. The equilibrium constant of 2MPy is approximately 60 times greater than its oxygen analog, 2HPy. The micro-ionization constants of the functional groups, -NH (p k A and p k C ) and -XH (p k B and p k D ), were determined to provide further insights into the ionization equilibria of these N-heteroaromatic XH compounds (2HPy and 2MPy). The relaxation time of water ( τ ) in aqueous solutions of 2HPy and 2MPy are collectively used with the K T values to determine the forward ( k f ) and backward ( k b ) rate constants of tautomerization. Subsequently, the k f and k b are used to provide the rationale for the K T and τ values.