Extending the Study of the 6,8 Rearrangement in Flavylium Compounds to Higher pH Values: Interconversion between 6‐Bromo and 8‐Bromo‐apigeninidin

The rearrangement between isomers 6‐ and 8‐bromo‐apigeninidin (6 and 8) was studied by pH jumps using stopped flow, UV/Vis, NMR, and HPLC analysis. The system constitutes a pH‐dependent network of chemical reactions involving up to 18 different species. The dynamic network is equivalent to a single diprotic acid exhibiting two pK (a)s, 2.55 and 5.4. Similar to other flavylium derivatives, the mole fraction of the species hemiketal and cis‐chalcone in both multistate isomers are negligible at the equilibrium. At pH 1, the pure isomers are slowly converted in a mixture containing about 50 % of isomers 6 and 8, while at pH 4, the system evolves to mixture of 10 % trans‐chalcone and 90 % of quinoidal bases. A series of pH jumps from pure isomer 6 at pH 1 to pH 6 and back to pH 1 leads to the same initial absorption spectra of the pure isomer 6. The same occurs for pure isomer 8, showing the lack of communication between the cis‐chalcones, at least in the time scale of few minutes. A pH jump from the equilibrated mixture of the isomers at pH 1.0 to 5.8 permits to follow a very slow isomerization.