Spectroscopic Determination of Acid Dissociation Constants of N-Substituted-6-acylbenzothiazolone Derivatives

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

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 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.

Acidity constant determination of novel drug precursor benzothiazolon derivatives including acyl and piperazine moieties

In this study, protonation and deprotonation behaviors of eight new drug precursor benzothiazolon derivatives in all of acidic and basic scale (super acidic, pH, super basic regions) are analyzed by using UV-visible spectrophotometric technique. Acidity constants (pKa), elucidation of the structure and protonation mechanisms of the studied molecules are obtained. Substituent effect on acidity constant values is discussed. These molecules are protonated from oxygen atom of acetamide group in the keto form. The protonation is found to be considerably contributed by the keto form.