Thione-thiol tautomerism and stability of 2- and 4-mercaptopyridines and 2-mercaptopyrimidines

The possible thione-thiol tautomerism of 2- and 4-mercaptopyridines, 2-mercaptopyrimidine, and 4,6-dimethyl-2-mercaptopyrimidine in solution is studied by means of absorption (UV-VIS) spectroscopy. In accordance with earlier observations, polar solvents and self-association shift the apparent tautomeric equilibrium significantly towards the thione form. In dilute solutions of nonpolar solvents the thiol form predominates. On standing, significant changes are observed in the absorption spectra of these tautomeric compounds in ethanol, dioxane, and water. The time course of the tautomerization, followed spectrophotometrically, reveals quantitative transformation of the thiol form to the corresponding symmetrical disulfides. The influence of concentration, temperature, and irradiation with indirect sunlight are discussed. This thione-disulfide process is reversible in water, starting either from the tautomeric thione or from its symmetrical disulfide, implying a possible importance in biological systems. Keywords: thione-thiol tautomerism, absorption spectra, thiol-disulfide oxidation, mercaptopyridines and pyrimidines.