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In this work, nine drug precursors, 2-(4,5-dimethyl-1-(phenylamino)-1H-imidazol-2-ylthio)-N-(benzothiazole-2-yl) acetamide derivative compounds, were newly synthesized by reacting 4,5-dimethyl-1-(phenylamino)-1H-imidazole-2(3H)-thione derivatives with 2-chloro-N-(thiazol-2-yl) acetamide compounds. Structures of the synthesized compounds were confirmed by 1H NMR, FTIR (Fourier transform infrared), MS (mass spectroscopy), and elemental analysis. The acidity constants of these acetamide derivatives were determined via UV spectroscopic studies. It was found that the first protonation of these compounds occurs on the nitrogen at imidazole ring, while the second protonation of these compounds occurs on the nitrogen at benzothiazole ring. The first pK a values were found to vary between 5.91 and 8.34 while the second pK a values were varying between 3.02 and 4.72.

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The acid dissociation constants of some 4-and/or 6-substituted 2-aminobenzothiazole and 4-substituted thiazole derivatives were determined spectroscopically. With the exception of a few, the first protonation was found to occur on the amino group. In some molecules where prototropic tautomerism is possible, a change-over in the protonation mechanism was observed. The first protonation under these circumstances was found to occur on the imino nitrogen atom. The second protonation takes place on the thiazole ring nitrogen atom.

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