New pH-dependent photosubstitution pathways of syringic acid in aqueous solution: Relevance in environmental photochemistry

The aqueous photochemistry of 4-hydroxy-3,5-dimethoxybenzoic acid (syringic acid) (1) and related compounds has been studied to investigate the novel pH-dependent photosubstitution pathways exhibited upon photolysis. Compounds of this type have previously been shown to produce chloromethane (CH3Cl) upon photolysis in chloride enriched aqueous solution. Photochemical product studies of 1 and related compounds, along with laser flash photolysis, and membrane introduction mass spectrometry (MIMS) – the latter to directly follow CH3Cl formation – were employed to study their photochemical behaviour. These studies revealed that the carboxylate form of 1 undergoes a previously unknown photochemical pathway that is initiated by excited state protonation of the benzene ring (ipso positions with OCH3 and OH substituents) by water. The enhanced basicity of the benzene ring at these positions is rationalized by an excited singlet state that has significant charge transfer from the carboxylate anion to the benzene ring, which is corroborated by semi-empirical AM1 (Chem 3D) calculations, as well as by the lack of reaction of the protonated form of 1 and related compounds incapable of this type of charge transfer. The photoproducts observed and/or isolated (CH3OH derived from the OCH3 group, 3-methoxygallic acid (2), 3,5-dimethoxybenzoic acid (4), and CH3Cl, the latter only when Cl− was added), can be explained by this new photochemical pathway.