Photo-oxidation of 2,4,6-trimethylphenol in aqueous laboratory solutions and natural waters: kinetics of reaction with singlet oxygen

2,4,6-Trimethylphenol (TMP) is rapidly oxidized by singlet oxygen in aqueous solution. The observed rate of this reaction can be described as the sum of two second-order processes, one for the phenol and one for the phenoxide anion. The corresponding second-order rate constants at 27°C are (6.2±1.0) × 10⁷ M^-1 s^-1 for the undissociated form of TMP and (1.10±0.02) × 10⁹ M^-1 s^-1 for the phenoxide anion. These values are consistent with previously reported quantitative structure-activity relationships based on half-wave potentials, E _{1 2}. However, unlike other phenols, the oxidation of TMP phenoxide by singlet oxygen appears to be reversible in aqueous systems with high pH, resulting in the reappearance of TMP with time after irradiation. Neutralization of the samples immediately after irradiation completely inhibits the back reaction. Similar results were obtained in solutions of humic acid and synthetic dyes which sensitize the formation of singlet oxygen. The unusual behaviour of TMP in these systems has been attributed to the formation of an endoperoxide which is not oxidized further, but for which the phenoxide undergoes significant thermolysis back to TMP.