Quantitative Model of Solvent Effects on Hydroxypyridine-Pyridone and Mercaptopyridine-Thiopyridone Equilibria: Correlation with Reaction-Field and Hydrogen-Bonding Effects

Article abstract of DOI:10.1021/jo01296a001

A model for the effect of reaction field and hydrogen bonding on the relative energies of protomers is applied to the equilibria between 6-chloro-2-hydroxypyridine and 6-chloro-2-pyridone, 2-mercaptopyridine and 2-thiopyridone, 6-chloro-2-mercaptopyridine and 6-chloro-2-thiopyridone, and 4-mercaptopyridine and 4-thiopyridone in a wide range of solvents.Quantitative correlation is obtained by a multivariable analysis.In addition to satisfactory statistical tests of the correlations, estimates of the differences in free energies between the isomers in the vapor phase and of the dipole moment component of the reaction-field term are obtained which compare well with the available independent values.These criteria are shown to signal an unacceptable correlation for the case of 2-chloro-4-hydroxypyridine and 2-chloro-4-pyridone.The advantage of this model, which provides an understanding of the effect of molecular environment on protomeric equilibria in terms of reasonable physical interactions, over empirical approaches is noted.