Abstract
The prototropic exchange equilibria of two drugs, nizatidine (I) and ranitidine (II), and also of structurally related the N, N′-dimethyl-2-nitro-1,1-ethenediamine molecule (III) were investigated. From the changes in electronic spectra in media of various acidity several protonation constants were determined. For nizatidine p K values were −0.82, 1.95, and 6.67; for ranitidine p K values were 1.95 and 8.13; and for III was 2.60. The hydroxylation equilibrium constant in strongly alkaline media was determined too. Corresponding p K a values were 13.23 for I, 13.26 for II and 13.76 for III. Molecular orbital calculations of electronic spectra confirmed that p K 1.95 for I and II, and p K 2.60 for III, are associated with C-protonation of nitroethenediamine fragment, while all p K a values correspond to the addition of HO − anion at the same double bond.
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