Dihydroxyquinoxaline Studies. Solubility, Ionization Constant, and Chelating Behavior

Abstract

Abstract The solubility, ionization constant, and chelating behavior of dihydroxyquinoxaline,. mainly 5,8-dihydroxyquinoxaline (5,8-DHQ) derivatives, have been measured and will be discussed. The introduction of a hydroxyl group into quinoxaline greatly lowers the solubility. The order of the decreasing solubility of 2,3-disubstituted 5,8-DHQ in water (at 20°C) is: O(CH2)2OC2H5<H<OH<CH3<C6H5<OC2H5<Cl<SH<SC2H5. For 5,8-DHQ the introduction of electron-repelling groups into 2,3-positions raises both the pKNH and pKoH values, while the introduction of electron-attracting groups into the same positions lowers both pKNHand pKoH. 5,8-DHQ, and its derivatives form colored precipitates with metal ions, but some derivatives which have pKNH values lower than zero fail to show any precipitation or coloration. 6,7-DHQ forms precipitate, and the precipitation colors distinctively with Cr(III) or Fe(II). The composition and stability constant (K) of copper(II) chelates of 5,8-DHQ and its derivatives in a dioxane-water (10 : 90 v/v) solution were studied spectrometrically. The copper chelate of 5,8-DHQ had a metal: ligand ratio of 1 : 1 and a logK value of 6.28 at pH 4.0. The structures of these copper chelates will be proposed.