Abstracts

Since its discovery by Chandross in 1963, the peroxyoxalate reaction has been widely studied using kinetic and mechanistic approaches and frequently utilized for analytical purpose. Most mechanism studies have been performed in anhydrous organic solvents; only recently partially aqueous media are more frequently utilized. Given the greater applicability of the aqueous medium for analytical purposes, in addition to better environmental compatibility, an aqueous medium was designed with an expected high light intensity emission for possible analytical applications of the reaction. The mechanistic studies under these reaction conditions were carried out by kinetics of the peroxyoxalate reaction in aqueous medium using phosphate buffer as the catalyst and fluorescein as the activator. Three oxalate esters of different reactivity, TCPO (bis(2,4,6-trichlorophenyl) oxalate), 2-MCPO and 3-MCPO (bis(2/3-methoxycarbonylphenyl) oxalate), were studied at pH values 6.0, 7.0, and 8.0, by variation of the hydrogen peroxide and phosphate buffer concentrations. The aqueous environment provides a large local polarity and multiple hydrogen bond possibilities, which influences the reactivity of the oxalate ester. Therefore, the reaction occurs much faster, but with emission intensities similar to that observed in anhydrous aprotic solvents. The observed rate constants (k_obs) show linear correlations with the hydrogen peroxide concentrations, indicating a bimolecular rate-limiting step with the participation of the peroxide and the oxalic ester concentration. Additionally, k_obs values show to be independent of the phosphate buffer concentration, indicating the occurrence of specific acid or base catalysis. Furthermore, the reaction constants increase with increasing pH values, pointing out that specific base catalysis is more efficient in these conditions. From the linear correlation of k_obs with the [H₂O₂] the hydrolysis and perhydrolysis rate constants, k_hyd and k_per can be obtained (k_obs = k_hyd + k_per [H₂O₂]). Interestingly, esters with poor phenolic leaving groups (2/3-MCPO) shows higher perhydrolysis rate constant than TCPO possessing a much better leaving group, as attested by the pK_a values of the corresponding phenols (pK_a values for phenolic leaving groups: 2,4,6-trichlorophenol: 6.1; 2-methoxycarbonylphenol: 9.53; 3-methoxycarbonylphenol: 9.05). The aqueous peroxyoxalate system is less efficient than in organic solvent, but with similar emission intensities. The reaction appears to involve fast proton transfer by water and the limit step contains one oxalic ester and one H₂O₂ molecule.