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Protonation constants and kinetics of hydrolysis of aroylhydrazones derived from nicotinic acid hydrazide

A series of aroylhydrazones were synthesized by condensation reaction of nicotinic acid hydrazide and differently substituted benzaldehydes. The protonation constants of hydrazones 1 to 12, as well as of starting compounds, were determined in methanol/water 1/1 mixture by spectrophotometric- potentiometric batch titrations. The electron- withdrawing substituents (–Cl and –NO2 groups) situated at 3 and/or 5 position on the salycilidene part of the hydrazone molecule lowered the K_1^H value assigned to protonation of amide nitrogen, and also the K_2^H value which refers to protonation of hydroxyl group at ortho position with respect to C=N double bond. In order to determine the effect of substituent on the stability of C=N bond, the kinetics of hydrolysis of hydrazones was studied spectrophotometrically in acidic and basic media. The substituents had significant effect on kinetics of hydrolytic processes. The electron-withdrawing substituents decreased the hydrolysis in acidic media, whereas the electron-donating groups (–OH, –OCH3) had the opposite effect. HPLC method was used for investigation of hydrolysis of compounds 4, 10, and 11, for which equilibrium between hydrolysis and condensation reaction was noticed. The observed rate constants, calculated from chromatographic and spectrophotometric data were in good agreement. Electrospray mass spectrometry was used for determination of reaction products.

Aroylhydrazones ; Protonation constants ; Kinetics ; UV-Vis spectrometry ; HPLC ; MS

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