Acidity constant and solvatochromic behavior of some pyrazolo[1,5-a]pyrimidin-2-amine derivatives Mohamed A. El-Gahami a,b,⇑ , Ahmed E.M. Mekky a,c , Tamer S. Saleh a,d , Abdullah S. Al-Bogami a a Chemistry Department, Faculty of Science, King Abdulaziz University, North Jeddah, P.O. Box 80203, Jeddah 21589, Saudi Arabia b Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt c Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt d Green Chemistry Department, National Research Centre, Dokki, Cairo 12622, Egypt highlights  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 k max exhibits a red shift in order cyclohexane ? CCl 4 ? CHCl 3 ? C 2 H 5 OH ? DMF. graphical abstract Electronic spectra, solvatochromic behavior and pKa values of some [1,5-a] pyrimidin-2-amine deriva- tives have been studied. Absorption spectra of 1.0  10 5 M compound VI in water (1) + methanol (2) at different pH’s. N N p- N NO N N (Form A) 2; N H2 m- CF3 R ; p +H+ N N N -F; -H+ N p- N N H Cl; p R Anionic Form ( Form C) -H; Tautomerism Forms HN N p N (Form B) -C N NH H3. N R article info Article history: Received 27 November 2013 Received in revised form 5 March 2014 Accepted 18 March 2014 Available online 28 March 2014 Keywords: Pyrazolo[1,5-a]pyrimidin-2-amine Solvatochromic behavior pKa determined abstract 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 k max 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-NO 2 < m-CF 3 < p-F < p-Cl < p-H < p-CH 3 . Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.03.029 1386-1425/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Chemistry Department, Faculty of Science, King Abdulaziz University, North Jeddah, P.O. Box 80203, Jeddah 21589, Saudi Arabia. Tel.: +966 564602431; fax: +966 2600376. E-mail address: malgahami@yahoo.com (M.A. El-Gahami). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 209–218 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa