Mechanism of bromination of a quaternary pyrimidinium cation. Change of rate determining step with acidity

Abstract

The kinetics of bromination of the 1,2-dihydro-1,3-dimethyl-2-oxopyrimidinium cation (Q+) in aqueous media (pH 0–5) have been studied using the stopped-flow method. At the higher acidities (pH < 2) the results are consistent with rate determining attack by bromine upon the pseudobase (QOH), whereas at low acidities (pH > 4) it appears that pseudobase formation is rate determining. The change occurs because at high acidity the reversal of the pseudobase QOH to the cation is fast relative to bromine attack, whereas at low acidity the converse is true. Results obtained at intermediate acidities (pH 2–4) are consistent with this interpretation.A separate kinetic study of pseudobase formation (and decomposition) yielded rate constants in good agreement with those derived from the bromination study.