Per the classification by Albert six-membered heterocycles can be described as π-deficient. Substitution by electronegative groups or additional nitrogen atoms in the ring significantly increase the π-deficiency. These effects also decrease the basicity.
Like pyridines, in pyrimidines the π-electron density is decreased to an even greater extent. Therefore, electrophilic aromatic substitution is more difficult while nucleophilic aromatic substitution is facilitated. An example of the last reaction type is the displacement of the amino group in 2-aminopyrimidine by chlorine and its reverse.
Electron lone pair availability (basicity) is decreased compared to pyridine. Compared to pyridine, N-alkylation and N-oxidation are more difficult. The pK value for protonated pyrimidine is 1.23 compared to 5.30 for pyridine. Protonation and other electrophilic additions will occur at only one nitrogen due to further deactivation by the second nitrogen. The 2-, 4-, and 6- positions on the pyrimidine ring are electron deficient analogous to those in pyridine and nitro- and dinitrobenzene. The 5-position is less electron deficient and substituents there are quite stable. However, electrophilic substitution is relatively facile at the 5-position, including nitration and halogenation.
Reduction in resonance stabilization of pyrimidines may lead to addition and ring cleavage reactions rather than substitutions. One such manifestation is observed in the Dimroth rearrangement.
Pyrimidine is also found in meteorites, but scientists still do not know its origin. Pyrimidine also photolytically decomposes into uracil under ultraviolet light.