Hydration of the Flavylium Ion

Article abstract of DOI:10.1021/ja00538a024

A spectral and kinetic investigation has been carried out of the transformations undergone in aqueous solution by the parent flavylium ion and its 4'-methyl and 4'-methoxy derivatives.Evidence is reported for the existence at some time under some condition of seven species, the flavylium ion (F⁺), two pseudobases, a 2-hydroxy adduct (B2) and 4-hydroxy adduct (B4), the cis-2-hydroxychalcone and its ionized form (cC and cC^-), and the trans-2-hydroxychalcone and its ionized form (tC and tC^-).At pH 6.5-8.5, F⁺ is relatively rapidly hydrated producing a mixture of B4, B2, and cC.The latter two are in equilibrium, their equilibration proceeding far more rapidly than F⁺ is hydrated.B4 is a kinetic product of the hydration only; over a short period of time it rearranges via F⁺ to the equilibrium mixture of B2 and cC.In base solution the behavior is similar, but the B2 <*> cC equilibrium is displaced toward the chalcone since it ionizes.Thus, at pH 12, F⁺ reacts very rapidly with OH^- producing a mixture of B4 and cC^-, the latter coming from B2 initially formed.This is followed by the rearrangement of B4, the kinetic product, to cC^-.In acid solutions, pH 2-6, an acid-base type equilibrium is relatively rapidly established between the cationic F⁺ and the neutral species B2 and cC.In all of these solutions, with the exception of strong acids, a slow further reaction occurs resulting eventually in complete transformation to tC or, in base, tC^-.The upper limit on the relative amount of cis-chalcone or pseudobase present in equilibrium with the trans-chalcone after complete reaction is 0.02percent.For a scheme B4<*>F⁺<*>B2<*>cC(<*>cC^-)-->tC(<*>tC^-), rate constants and equilibrium constants for each reaction stage have been obtained by a kinetic and spectral analysis.