Structural changes of flavylium salts. IV. Polarographic and spectrometric examination of pelargonidin chloride

Abstract

Pelargonidin chloride has been examined polarographically and spectrometrically in aqueous methanol over a wide pH range and the molecular changes determined. In strongly acid solutions the flavylium ion is stable, but between pH3 and pH8 ring opening to the α-diketone occurs, the process involving the pseudo-base and its 3-keto tautomer. The pseudo-base is present in the equilibrium mixture from pH3 to pH12 in amount varying with the pH. The anhydrobase, detected in the equilibrium mixture at pH6, is stable above pH7, becoming ionized as the pH is raised. Unlike the anhydrobase of the simpler flavylium salts, the anhydrobase of pelargonidin chloride does not undergo ring cleavage in strong alkali under the conditions of these experiments. Ionization of the phenolic group at C5 was indicated by a, small anodic wave in the polarograms above pH9.5, due to mercuration at C6. A similar reaction, analogous to the known catalytic mercuration of ketones, was observed for phloroglucinol and resorcinol in alkaline solutions.