The kinetics and mechanism of electrophilic substitution of heteroaromatic compounds. Part XV. Hydrogen exchange of 3,5-dimethylphenol and heterocyclic analogues

Abstract

Acid-catalysed hydrogen exchange in the 3- and 5-positions is shown to occur on the free-base species of the 2,6-dimethyl derivatives of 4-pyridone, 1-hydroxy-4-pyridone, 4-pyrone, and 4-thiapyrone. At higher acidities, the mechanism changes for the first two compounds which then undergo exchange on the conjugate acids, and in addition 1-hydroxy-2,6-dimethyl-4-pyridone reacts at low acidities as its conjugate base. The mechanism of simultaneous methyl-group exchange in the pyrone and thiapyrone is elucidated. The kinetic data are used to determine the quantitative effect of the hetero-groups NH^+–, N⁺(OH)^–, N⁺(O^–)^–, O^+–, and S^+– on the rates; σ_m⁺ values are derived and discussed.