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

A 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.