Acid-Base Properties Of Alpha,Beta-Unsaturated Ketones Obtained From Reaction Of 5-Formyl-8-Hydroxyquinoline With P-Substituted Acetophenones

Chalcones (alpha,beta-unsaturated ketones) containing aromatic or heterocyclic radicals are highly reactive, allowing the synthesis of novel organic compounds. In this study, the dissociation constants (pKa) of seven chalcones derived from 8-hydroxyquinoline were determined and the influence on dissociation of substituents in the phenyl group (-CH3, -OCH3, -N(CH3)(2), -Cl, -Br, and -NO2) was analysed. pKa values are important because they determine the pH at which ligands are fully deprotonated -when they show their maximum chelating properties-and determine the ligands interactions at different pH values. The chalcones' pKa's were calculated by visible ultraviolet spectroscopy in a water-ethanol (1: 1) mixture using the Henderson-Hasselbach equation. It was shown that the 8-hydroxyquinolinic fragment has a large electron donor effect on the pi system of the chalcones. The introduction of substituents (R) in the phenyl fragment of the chalcones slightly affected the dissociation of the hydroxyl group and the protonation of the nitrogen in the hydroxyquinoline fragment. The acceptor substituents (Cl, Br, NO2) increased the polarity of OH- and its acidity. Nitrogen protonation decreased electron donor properties of this fragment, and deprotonation of the hydroxyl caused the opposite effect. Substituents introduction in the phenyl fragment slightly affected hydroxyl group dissociation and nitrogen protonation.