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Spectroscopic Determination of Dissociation Constants of Some 4- nitrobenzaldehyde-4-substituted phenyl-1-carbonylhydrazones in Sodium Hydroxide Media

Author(s):

Mirjana S. Jankulovska*, Ilinka Spirevska, Vesna Dimova and Milena Jankulovska   Pages 1 - 10 ( 10 )

Abstract:


Purpose: Hydrazones are a class of azomethines with a wide spectrum of pharmacological properties which are influenced by pH of the media. The purpose of this study was investigation of acid-base properties of five 4-nitrobenzaldehyde-4-substitutedphenyl1-carbonylhydrazones in sodium hydroxide media (14>pH>7).

Method: The dissociation process was followed by UV-Vis spectroscopy, in the 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 dissociation process for four investigated hydrazones with 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 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 amide group of this hydrazone was retarded due to the influence of phenolic group.

Keywords:

hydrazones, dissociation, dissociation constants, deprotonation enthalpy, UV-Vis spectroscopy, semiempirical methods.

Affiliation:

Faculty of Agricultural Sciences and Food, Ss. Cyril and Methodius University in Skopje, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University in Skopje, Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University in Skopje, Chemical-Bromatological-Toxicological Analysis Laboratory, Higher Medical School, Ss. Kliment Ohridski University in Bitola



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