Computational Analysis of Salicylic Acid Oxidation: Byproducts and Reaction Pathways

This study used density functional theory to study the reaction pathways and intermediates associated with the oxidation of salicylic acid (SA). The analysis was carried out for each of the three SA charge states (i.e., 0, -1, -2) that are observed in aqueous solution. SA can be oxidized to the three primary byproducts, 2,3 DHBA, 2,5 DHBA, and catechol, via cyclohexadienyl radical intermediate formation. A second mechanism consists of an initial H• radical abstraction, which produces organic radicals that react with hydroxyl radicals, but this pathway does not form a catechol. Activation energy calculations confirmed that 2,3 DHBA is the most thermodynamically favored byproduct, followed by 2,5 DHBA and catechol. Formation of uncommon byproducts was also investigated. The operational implications for water treatment processes are discussed. To the author's knowledge, this is the first study to use the electronic properties of SA to elucidate the oxidation pathways across the full range of pH values found in water.