The kinetics and mechanism of electrophilic substitution of heteroaromatic compounds. Part XIX. Acid-catalysed hydrogen exchange of some pyrimidine derivatives

Abstract

2-Amino- and 6-amino-2,4-dimethyl-pyrimidine undergo hydrogen–deuterium isotope exchange as free bases at the 5-position in dilute acid. The latter can also exchange as the conjugate acid, but both are hydrolysed in concentrated acid. Rate constants for the exchange are correlated by the Hammett linear free energy treatment. Pyrimidin-2-one undergoes exchange by way of a small proportion of the covalent hydrated species : this mechanism is supported by a study of the 1,2-dihydro-1,3-dimethyl-2-oxopyrimidinium ion and other derivatives. General equations have been derived for the kinetics of acid-catalysed hydrogen exchange of bases.