Photochemistry of the three carboxypyridines in water: a pH dependent reaction
The photochemistry of ortho, meta and para-carboxypyridines (pKa1 = 1.0–2.1 and pKa2 = 4.7–5.3) in aqueous medium was studied by laser-flash photolysis and product studies. At pH < pKa1, hydroxylated compounds are produced with low quantum yields. Within the pH range 4–7, ortho and meta isomers undergo dimerization together with decarboxylation with a quantum yield showing a very sharp maximum around pKa2 (ϕmax = 0.09 and 0.01, respectively) while the para isomer is photostable. End-of-pulse transients assigned to triplet states were detected by laser-flash photolysis at pH < pKa1 and pH > 4. Additionally, the carboxypyridinyl radicals were detected as secondary intermediates at pH < pKa1 and 4 < pH < 7 and the OH-adduct radicals at pH < pKa1. This is in favour of an electron transfer reaction between triplet and starting compound producing a charge transfer species. The radical anion would escape as carboxypyridinyl radical while the radical cation may add water at pH < pKa1 yielding the OH-adduct radical or may undergo decarboxylation at pH > 4. The high quantum yield of phototransformation of the ortho isomer at pH > 4 is due to an easy decarboxylation process. A reaction scheme is proposed accounting for the dependences of ϕ on both the pH and the carboxypyridines concentration. This study points out the distinct pattern of reactivity of carboxypyridines depending on the ionisation state of starting compounds and isomeric substitution.