Hydration of the Flavylium Ion. 2. The 4'-Hydroxyflavylium Ion

Article abstract of DOI:10.1021/jo00140a027

An investigation of the transformations undergone by the 4'-hydroxyflavylium ion in aqueous solution is reported.A rapid initial deprotonation of the hydroxy group occurs, resulting in an anhydro base, the pK_a value associated with this ionization being 5.5.A slower transformation follows in which further equilibration occurs with a pseudobase and ring-opened cis-chalcone.The stabilities of the neutral species are very similar, the mixture at equilibrium consisting of 37percent pseudobase, 33percent neutral chalcone, and 30percent anhydro base.This equilibrium is displaced toward flavylium ion in acid solutions (pH <5) and is displaced toward chalcone in base due to the ionization of its phenolic hydroxyl groups.The pseudobase is formed in solutions with pH <8 by hydration of the flavylium ion, while in solutions with pH >8 direct hydration of the anhydro base occurs.In all solutions with pH >4 a very slow further reaction occurs, resulting in the irreversible formation of trans-2,4'-dihydroxychalcone, the product of thermodynamic control at all pHs.