Basicity and alkaline hydrolysis of 1,2-diaryl-1,4,5,6-tetrahydropyrimidines. Application of the Hammett equation

Abstract

The pK_a values of nine 1,2-diaryl-1,4,5,6-tetrahydropyrimidines have been determined spectrophotometrically. The influence of substituents at N-1 and C-2 upon basicity has been studied. When the tetrahydropyrimidine ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. The stability of these compounds to hydrolysis in boiling alkaline 95% ethanol was studied. Reaction rates were enhanced by electron-releasing phenyl substituents at N-1 and reduced by electron-withdrawing groups. Agreement with the Hammett equation allowed calculation of rate constants. The observed and calculated rate constants of hydrolysis are nearly equal. An equation relating the rate constants with the ionization constants of tetrahydropyrimidinium ions is given.