Acidity constant and solvatochromic behavior of some pyrazolo[1,5-a]pyrimidin-2-amine derivatives

•Novel pyrazolo[1,5-a]pyrimidin-2-amine derivatives have been synthesis and characterized.•pKa values depended largely on both the amount and basicity of organic solvent.•The λmax exhibits a red shift in order cyclohexane → CCl4 → CHCl3 → C2H5OH → DMF.

The UV–visible electronic spectra of some azo compounds of pyrazolo[1,5-a]pyrimidin-2-amine have been studied. The solvatochromic behavior of these compounds was investigated by studying their spectra in pure organic solvents of different polarities such as cyclohexane carbon tetrachloride, chloroform, ethanol and DMF. These exhibits a red shift in its λmax with increase relative permittivity of medium changing from cyclohexane → carbon tetrachloride → chloroform → ethanol → DMF. The acid dissociation constants of these compounds were determined in aqueous–organic solvent mixtures such as acetone, methanol, ethanol and DMF. The ionization constants of the dyes in question depend largely on both the proportion and the nature of the organic solvent basicity contribute the major effects on the ionization process. In general, pKa values in all compounds decrease with increase relative permittivity of the medium. The acidity of studied azo compounds increases in the following order:p-NO2 < m-CF3 < p-F < p-Cl < p-H < p-CH3.

Graphical abstractElectronic spectra, solvatochromic behavior and pKa values of some [1,5-a] pyrimidin-2-amine derivatives have been studied. Absorption spectra of 1.0 × 10−5 M compound VI in water (1) + methanol (2) at different pH’s.Figure optionsDownload full-size imageDownload as PowerPoint slide