Reactions of the flavonoid hesperetin with chlorine: a spectroscopic study of the reaction pathways

The flavonoid hesperetin (Hsp) contains aromatic rings substituted with hydroxyl and methoxyl groups, which activate it toward electrophilic attack and hence make it a potential surrogate for natural organic matter with respect to reactions with chlorine. This paper describes the chlorination pathway of Hsp, based on a combination of electrospray tandem mass spectrometry (MS/MS) and absorbance spectroscopy. When a solution containing Hsp is dosed with NaOCl at pH 7, chlorine substitution into Hsp occurs exclusively into the meta-dihydroxy substituted ring. The first two Cl atoms to enter the ring do so at the highly activated carbons that are each ortho to two oxygenated carbon atoms. These substitutions make the molecule more acidic, but do not change its primary structure or aromaticity. The third Cl atom that substitutes into the molecule does so at one of the previously chlorinated sites, destroying the aromaticity of the ring and altering the molecular properties more dramatically than do the first two. The absorbance spectra of Hsp and mono- and di-chlorinated Hsp are all very similar and are very distinct from that of trichlorinated Hsp. In particular, the latter is the only one of those species that absorbs visible light (in a characteristic band centered at approximately 422nm). Di- and trichloroHsp form even at low molar Cl/Hsp ratios, and can coexist with Hsp and monochloroHsp in neutral, aqueous solutions for at least 24 h in the absence of free chlorine. If free chlorine is present, the less-chlorinated species continue to acquire Cl, and trichloroHsp undergoes further reaction, until either the free chlorine or the trichloroHsp is fully depleted. The appearance of di- and trichloroHsp while substantial amounts of Hsp remain unreacted indicates that substitution of one or two Cl atoms into the ring facilitates the incorporation of yet more Cl into the ring. This autoacceleration of the chlorination process is hypothesized to be induced by the increase in acidity that accompanies Cl incorporation. Specifically, the increase in the acidity of the phenolic groups shifts the equilibrium toward the enolate anion, which is considered to be much more amenable to electrophilic attack than the enol.