Spectroscopic Study of the Basicity of 4,6-Dihydroxypyrimidine Derivatives

The protonation of a number of 4,6-dihydroxypyrimidine derivatives is studied, and the features of the electronic spectra of free bases and protonated forms are considered. It is shown that the alkyl substituents in position 2 increase the basicity of the compound, and the nitro group in position 5 leads to its decrease. In an acid medium (0.1–99.5% H₂SO₄), 4,6-dihydroxypyrimidine, 6-hydroxy-2-methylpyrimidine-4­(3H)-one, and 6-hydroxy-2-ethylpyrimidine-4­(3H)-one have two protonation stages, barbituric acid is protonated in three stages, and 6-hydroxy-2-methyl-5-nitropyrimidine-4­(3H)-one and 6-hydroxy-2-ethyl-5-nitropyrimidine-4­(3H)-one form a monocation.