抄録
Protonation equilibrium of a series of 1, 2, 3, 4-tetrasubstituted 3-pyrazoline-5-thiones (I) was examined by spectrophotometry and potentiometric titration. In aqueous solution of sulfuric acid, protonation of the sulfur atom in the thiopyrazolone ring of I occurs and the increase in their ionization rate is greater than the increase of Hammet's acidity function, h0, by increasing concentration of sulfuric acid. This phenomenon can be explained by the highly polarized structure of I resulting in the different degree of hydration between I and the Hammet indicator. Basicity of I agrees with that presumed from the I-effect of the substituent, and the basicity of 4-substituted thiopyrine derivatives fall in the order of CH3, H, C6H5, I, Br, Cl, NO2, NH3+, and NH(CH3)2+ as a substituent. Ionization constant of thiopyrine is -0.35 and it is a much weaker base than antipyrine.
