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

The basic ionization (pk(1)) and acidic ionization (pk(2)) constants and equilibrium constant (K(T)) of 2HPy and 2MPy were determined. The pk(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 (pk(A) and pk(C)) and –XH (pk(B) and pk(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.