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Protonation constants of aromatic hydrazones

The protonation constants of aroylhydrazones derived from nicotinic acid hydrazide (Figure 1) were determined in methanol/water 1/1 mixture at 25 °C and ionic strength 0.1 mol dm– 3 (NaCl). Aroylhydrazones were prepared according to the procedures described elsewhere [1]. Considering the fact that aroylhydrazones hydrolyse in water-containing solvents, batch spectrophotometric titrations were carried out. All UV-Vis spectra were recorded within first 3 min following the preparation of solutions. To provide more accurate assignation of the pK values for aromatic hydrazones, comparison with the protonation constants of starting compounds, which were also determined in this work, was made. The pK values assigned to the protonation of amide nitrogen were in the range from 10.5 to 12.9, whereas those corresponding to the ortho-hydroxyl group ranged from 6.4 to 8.9. Constants corresponding to the protonation of pyridine nitrogen (which takes place at pH ≤ 2) could not be determined due to the rather fast hydrolysis.

aroylhydrazones ; UV-Vis ; protonation constants

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