Aqueous OH Kinetics of Aliphatic Carboxylic Acids: New Data and Updated Structure-Activity Relationship

The reactions of carboxylic acids and their conjugate bases with the hydroxyl radical (OH) play an important role in atmospheric multiphase chemistry, yet their kinetics remain poorly constrained. Here, we report new aqueous-phase rate coefficients for 22 linear, branched, and cyclic C₄-C₁₀ monocarboxylic acids measured between 278 and 323 K with a relative rate technique using 1-propanol, 1-butanol, and 1-pentanol as kinetic reference compounds. The obtained data were combined with literature values to construct the comprehensive kinetic data set, containing 112 rate coefficients for 61 carboxylates and 51 undissociated acids. Activation parameters (activation energy, enthalpy, entropy, and Gibbs free energy of activation) were compiled for 106 species, revealing systematic correlations with molecular structures, protonation state, and carbon chain length, highlighting the role of solvation and hydrophobic interactions. Based on this data set, an improved group-contribution (Atkinson's) structure-activity relationship model was developed, which, for the first time, incorporating direct OH reactions with carboxylic and carboxylate moieties. The updated structure-activity relationship reproduces measured rate coefficients with higher accuracy than previous models, indicating a non-negligible contribution of acidic hydrogen abstraction and single-electron transfer pathways. At the same time, the refined structure-activity relationship performed poorly for highly oxygenated and poly(carboxylic) acids, indicating the need to modify the framework of the group-contribution structure-activity relationships used in the field of aqueous kinetics, focusing on OH reactions with water-soluble organic compounds.