Substituent effects on the pH‐dependent multiequilibria of flavylium salt analogs of anthocyanins

Substituent effects on the hydration, tautomerization, and isomerization equilibria of flavylium salts can be described by a series of linear free energy relationships (LSER) based on Hammett correlations. The positions on the flavylium rings were classified as either activated ( para ‐like) or nonactivated ( meta ‐like) to decide which σ value to employ ( σ R or σ m , respectively), while the steric effects of substituents at C‐3 were included via the E S parameter. Based on these relationships, we then show that it is possible to predict values of the “apparent p K a ” (p K ap ) of flavylium ions that were not included in the original data set, as well as those of several naturally occurring anthocyanins. The value of p K ap provides a measure of the thermodynamic stability of the flavylium cation as a function of pH and is directly related to the pH range in which the color of the flavylium cation form of anthocyanins persists in aqueous solution. Copyright © 2011 John Wiley & Sons, Ltd.