Aqueous OH kinetics of saturated C6-C10 dicarboxylic acids under acidic and basic conditions between 283 and 318 K; new structure-activity relationship parameters

Bimolecular reaction rate coefficients (k(OH)) for reaction of hydroxyl radicals (OH) with adipic, pimelic, suberic, azelaic, sebacic and cis-pinonic acids were measured in aqueous phase for the neutral (pH = 2) and completely deprotonated (pH = 9) forms of the acids listed. Reactions were carried out in bulk solution between 283 and 318 K. Camphoric acid was used as a reference compound for measuring kOH values using a relative rate method, and with hydrogen peroxide (H2O2) as a photochemical source of OH. Concentrations of the reactants were monitored with liquid chromatography coupled with electrospray tandem mass spectrometry (LC-ESI/MS2). The kOH values measured were between 1 and 8 x 10-9 M(-1)s(-1);only adipic and pimelic acids showed a significant pH-dependence to their OH reactivity. The kinetic data acquired and the data reported in the literature for camphoric, glutaric and succinic acids were used to optimize new structure-activity relationship (SAR) parameters. The SAR developed here allowed accurate prediction of kOH values for aliphatic carboxylic acids in the temperature range between 283 and 318 K. The accuracy of the developed SAR was evaluated against the experimental kOH values measured in this work and obtained from the literature data. Moreover, this work is the first to accurately predict the temperature-dependent kOH values for carboxylic acids.

Automated summary

This study measured the reaction rate coefficients of hydroxyl radicals with various forms of aliphatic carboxylic acids, identifying significant pH dependence in only some acids. By optimizing structure-activity relationship parameters, accurate prediction of kOH values for aliphatic carboxylic acids in a specific temperature range was achieved. The developed SAR model was validated against experimental measurements from this study and literature data.