Kinetics and mechanism of the bromination of 4-pyridone and related derivatives in aqueous solution

Abstract

The kinetics of bromination of 4-pyridone and selected derivatives have been measured in aqueous solutions in the pH range 0–9, at 25 °C. The tautomeric system 4-pyridone [Formula: see text] 4-hydroxypyridine [Formula: see text] reaets with bromine via the predominant (pyridone) tautomer at pH < 6 and via the conjugate anion at pH > 6. 3-Bromo-4-pyridone behaves similarly. The kinetics also reveal that the facile dibromination of 4-pyridone occurs because at most pH's the monobromo derivative is actually more reactive towards bromine by virtue of its lower pK a values. From the point of view of reactivity the 4-pyridones and their anions behave as substituted phenoxide ions. 4-Methoxypyridine does not undergo bromination under comparable conditions, but rather forms a complex with bromine.