Purpose: Hydrazones are a class of azomethines with a wide spectrum of pharmacological
properties that are influenced by the pH of the media. The purpose of this study was to investigate acidbase
properties of five 4-nitrobenzaldehyde-4-substitutedphenyl-1-carbonylhydrazones in sodium hydroxide
Method: The dissociation process was followed by UV-Vis spectroscopy, in ethanol-water (V/V, 1:1)
solutions, at room temperature. Semiempirical methods AM1 and PM3 were applied for determination
of the deprotonation enthalpies.
Results: The changes in the UV-Vis spectra, as well as the deprotonation enthalpies, suggested that the
dissociation process for four investigated hydrazones with an amide group took place in one step. The
exception with two dissociation steps was hydrazone with amide and hydroxyl group. The pH region of
dissociation was from pH 10.8 to pH 11.6 for the first step and between pH 11.7 and pH 12.1 for the
second step of dissociation. The influence of the ethanol on the UV-Vis spectra was eliminated by the
method of Characteristic Vector Analyses (CVA). The stoichiometric dissociation constants were determined
numerically (pKHA = n·pH + logI) and graphically (intercept of the dependence of logI on
pH) from the absorbance data using experimental and reconstructed UV-Vis spectra, at three different
ionic strengths. Thermodynamic dissociation constants were estimated graphically as an intercept of
dependence of dissociation constant on the square root of the ionic strength.
Conclusion: The obtained results demonstrated that the influence of the substituents on pKHA values
was not significant, except for hydrazone with amide and hydroxyl group. Namely, the dissociation of
the amide group of this hydrazone was retarded due to the influence of the phenolic group.