Electrochemical Evaluation of the Equilibrium Constant of Keto-enol Tautomerism in Piroxicam

Abstract

Piroxicam(I) was found to exhibit keto-enol tautomeric equilibrium between the keto(1A) and enol(1B) forms in aqueous media with the enol form as the predominant species (about 80%). Making use of the electroanalytical methods, differential pulse polarography and cyclic voltammetry, the keto-enol tautomeric equilibrium constant, K_t, was evaluated. Below pH 5, it was found that pH had hardly any effect on the equilibrium constant suggesting that the proton exchange across the keto and enol forms to be taking place via an intramolecular hydrogen bond. In basic pH's, due to possible deprotonation and other accompanying acid-base equilibria the keto-enol tautomerism was observed to be disturbed. The K_t value (C_keto/C_enol) is found to be 0.224 while the average diffusion coefficient of piroxicam as 1.2020 x 10^–6cm² s^–1.