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

Abstract

The basic ionization (pk₁) and acidic ionization (pk₂) constants and equilibrium constant (K_T) of 2HPy and 2MPy were determined. The pk₁(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.