Investigation of Dissociation Process of Some 4-Methoxybenzaldehyde Benzoylhydrazones in Sodium Hydroxide Media by UV Spectroscopy

The importance and wide application of hydrazones are due to a broad spectrum of biological activities that they possess. The biological activity generally depends on the pH values of the media which have influence on the extent of ionization of organic compounds. The dissociation process of five 4-methoxybenzaldehyde benzoylhydrazones in aqueous sodium hydroxide media was determined using UV spectroscopy method, at room temperature. The influence of the substituents in the pposition of benzoyl moieties was investigated. Prediction of the deprotonation site in the hydrazone molecule was made by using the deprotonation enthalpy values calculated using the AM1 and PM3 semiempirical methods. The dissociation process took place in one step as a result of the dissociation of the amide group, or in two steps for 4-methoxybenzaldehyde 4-hydroxybenzoylhydrazone. The changes in the UV spectra with the basicity of the media were used for calculation of the dissociation constant values. One dissociation constant (pKHA) was calculated for hydrazones which contain only an amide group. The first and the second dissociation constants (pKH2A and pKHA −) were determined as a result of the dissociation of the amide and phenolic group of 4-methoxybenzaldehyde 4- hydroxybenzoylhydrazone which contain these groups. The dissociation of amide group of this hydrazone was retarded due to the influence of phenolic group. In order to obtain thermodynamic pKHA values, the measurements were performed at three different ionic strengths (0.10, 0.25, and 0.50 mol/dm3). The first deprotonation site was due to dissociation of phenolic group, while the second one was a result of dissociation of the amide group. The strength of the acid decreased by following the order: 4- chlorobenzoyl- > benzoyl- > 4-methoxybenzoyl- > 4-methylbenzoyl- > 4-hydroxybenzoyl hydrazone. The obtained results demonstrated that the influence of the substituents was not significant, except for 4-methoxybenzaldehyde 4-hydroxybenzoylhydrazone.