Dissociation constants of hydroxybenzoic acids revisited

Journal of Molecular Liquids, Volume 442, 15 January 2026, 129119.

Dissociation constants of hydroxybenzoic acids revisited: influence of phenol groups quantity and position on pKa

https://doi.org/10.1016/j.molliq.2025.129119

Abstract:

This paper reports a study of the acid-base equilibria of benzoic acid and twelve of its hydroxylated derivatives: salicylic, 3-hydroxybenzoic, 4-hydroxybenzoic, 2,3-dihydroxybenzoic, 2,4-dihydroxybenzoic (resorcylic), 2,5-dihydroxybenzoic (gentisic), 2,6-dihydroxybenzoic (γ-resorcylic), 3,4-dihydroxybenzoic (protocatechuic), 3,5-dihydroxybenzoic (α-resorcylic), 2,3,4-trihydroxybenzoic, 2,4,6-trihydroxybenzoic, and 3,4,5-trihydroxybenzoic (gallic) acids. The dissociation constants (of the single bondCOOH groups) were determined spectrophotometrically at 298.2 K, 0.1 MPa, and an ionic strength of 0.05 M (NaClO4). A simple incremental model was developed to correlate the pKa values and the positions of the absorption maxima in the UV–vis spectra with the number and position of the hydroxyl substituents. To elucidate the influence of the single bondOH groups on the electronic absorption spectra and to identify predictive markers for pKa, density functional theory (DFT) calculations were performed at the B3LYP-D3(0)/aug-cc-pVTZ level. These calculations employed a mixed implicit-explicit solvation model to compute the pKa values and electronic spectra. The Hirshfeld charges on the oxygen atoms of the single bondCOO− groups in the deprotonated anions and the –COOH groups in the neutral acids were identified as key predictors of acidity.

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