Tomislav Benković

Hydrolysis and Extraction Properties of Aroylhydrazones Derived from Nicotinic Acid Hydrazide

The kinetics of hydrolysis of N′-salicylidene-3-pyridinecarbohydrazide (1) and N′-(2-hydroxy-3-methoxyphenylmethylidene)-3-pyridinecarbohydrazide (2) were investigated in the dioxane/water (1/1, volume ratio) solvent system in acidic and basic media, and the corresponding reaction mechanisms were proposed. The kinetic results suggest that in the acidic solutions the rate-determining step of the hydrolysis reaction is the attack of a water molecule on the protonated azomethine group, whereas in the basic medium it is the attack of water on the completely deprotonated hydrazone. By analyzing the temperature dependence of the reaction rates, the thermodynamic activation parameters, i.e. activation enthalpy and entropy, were also evaluated and discussed. The complexation reactions of vanadium(V) with aroylhydrazones 1 and 2 as well as the extraction of V(V) from aqueous to an organic phase were studied spectrophotometrically. The optimum conditions for complex formation and extraction of vanadium(V) into chloroform solutions of compounds 1 or 2 have been evaluated. Based on the results obtained by Job’s method and the equilibrium shift method, the stoichiometries of the complexes were determined to be 1:1. The effect of foreign ions on V(V) extraction by compound 2 has been determined as well.

Acid-base Properties and Kinetics of Hydrolysis of Aroylhydrazones Derived from Nicotinic Acid Hydrazide

AbstractA series of aroylhydrazones were synthesized from nicotinic acid hydrazide and differently substituted benzaldehydes. The protonation constants of the 12 resulting hydrazones, as well as of the starting compounds, were determined in methanol/water 1/1 mixtures by spectrophotometric–potentiometric batch titrations. In order to determine the effect of substituents on the stability of the C=N bond, the kinetics of hydrolysis of hydrazones was studied spectrophotometrically in acidic and basic media. The HPLC method was used for investigation of the hydrolysis of selected compounds, for which equilibrium between hydrolysis and the condensation reaction was observed. The observed rate constants, calculated from chromatographic and spectrophotometric data, are in good agreement. Electrospray mass spectrometry was used for determination of reaction products. Oxidation and formation of quinones in basic media were observed for the 2,3- and 2,5-dihydroxy substituted benzaldehyde derivatives. On the other hand, the 2,4-dihydroxy derivative showed higher stability compared to N′-salicylidene-3-pyridinecarbohydrazide.