Acid-Base Equilibria of Some N-Substituted Thiophene-2-Carboxamidoximes in Non-Aqueous Media

Potentiometric titration in non-aqueous media is a standard method for the determination of the basicity and acidity of various compounds, particularly in organic and pharmaceutical analyses. The reason for using non-aqueous solvents is that many organic compounds of pharmaceutical importance do not dissolve in water. Furthermore, since water is amphoteric, only a limited range of acid and base strengths can be determined in this solvent. Amidoximes are compounds with both a hydroximino and an amino functionality at the same carbon atom, and are thus closely related to amides, amidines and hydroxamic acids. Literature searches revealed that a remarkable number of amidoximes have been found to have important biological activities including anti-tuberculostatic, anti-thrombotic and vasodilating, anti-malarial, anti-depressive and alpha-adrenergic3−9. As a continuing part of our studies on the acid-base equilibria of the amidoximes and related compounds, we report here the protonation of the amino nitrogens of six N-substituted thiophene-2carboxamidoximes, of which four are new, (Scheme 1) in acetic acid by potentiometry.