J. Iran. Chem. Soc., Vol. 8, No. 2 June 2011, pp.502~512.

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A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium

M. Jankulovskaa,*, I. Spirevskaa and V. Dimovab

aInstitute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts Cyril and Methodius University, Arhimedova 5, MK-1001 Skopje, Republic of Macedonia
bFaculty of Technology and Metalurgy, Sts Cyril and Methodius University, P. O. Box 580, MK-1001 Skopje, Republic of Macedonia

     The spectral behavior of some benzaldehyde hydrazones was examined via the UV spectroscopic technique at room temperature and in the pH region between 1 and 7 in aqueous perchloric acid medium. The acid-base equilibrium was characterized qualitatively and quantitatively. It was found that the protonation reaction took place. The spectra of the solution of benzaldehyde hydrazones at different pHs were studied and utilized for the determination of ionization constants of the protonated forms of hydrazones (pKBH+). The pKBH+ values were determined numerically and graphically from the absorbance values of the experimental and reconstructed spectra by characteristic vector analysis. In order to obtain thermodynamic pKBH+ values, measurements were performed at ionic strengths of 0.1, 0.25 and 0.5 M (NaClO4). There was a good agreement between the obtained pKBH+ values of the investigated hydrazones and those of similar classes of compounds. The site of protonation of the investigated hydrazones was also studied, too. The proton affinities for the different nitrogen atoms of the hydrazone molecule were calculated using AM1 and PM3 semiempirical methods. It was demonstrated that the protonation occurred at the imino nitrogen atom of hydrazone molecule. The effect of the chemical structure on the ionization constants was also examined.

Keywords: UV spectroscopy, Protonation, Thermodynamic ionization constants, Benzaldehyde hydrazones, Semiempirical methods

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