Ultraviolet-visible study on acid-base equilibria for some 7,8-ethylenedioxy coumarins

Abstract

The UV-vis spectra of recently synthesized 7,8-ethylenedioxy coumarin, (I), and 7,8-ethylenedioxy-4-methyl coumarin (II), were studied in 5% v/v aqueous methanol. The nature of the electronic transitions and the role of methyl group of II in the behavior of UV-vis spectra were discussed. Acid-base equilibria of the compounds against varying pH have been examined in detail. Acid dissociation constants (pK(a)) were determined at an ionic strength of 0.10 M by using the Henderson-Hasselbalch equation. The mean acidity constants for the protonated forms of the compounds were determined as pK(a) = 10.492 (lambda = 259.2 nm) for I and pK(a) = 10.794 (lambda = 258.2 nm) for II. Based on UV-Vis spectra, the preferred mechanisms for base catalyzed hydrolysis and dissociation reactions were discussed. A mechanism showing intramolecular charge transfer character of the studied molecules was also proposed. (C) 2013 Elsevier B.V. All rights reserved.