Are Benzoic Acids Always More Acidic Than Phenols? The Case of ortho-, meta-, and para-Hydroxybenzoic Acids

To address the title question, the relative intrinsic acidities of phenol and benzoic acid as well as the isomeric family of ortho-, meta-, and para-hydroxybenzoic acids were compared. Dissociation of the [PhCO2HOPh](-) proton-bound dimer showed slightly greater acidity for benzoic acid. Using traveling-wave ion mobility mass spectrometry (TWIM-MS) with CO2 as the drift gas and post-TWIM collision-induced dissociation, the gaseous deprotonated molecules of the isomeric hydroxybenzoic acids were properly separated and characterized. For the para isomer, an intrinsic gas-phase acidity order inverse to that in solution was found, as before, that is, the phenol site of para-hydroxybenzoic acid was found to be considerably more acidic than its benzoic acid site, whereas the opposite was observed in solution, regardless of the solvent. However, for the ortho and meta isomers, the inversion in acidity order upon going from solution to the gas phase was not observed, and gaseous carboxylate anions were still found to predominate over phenoxide anions. Actually, for the ortho isomer, an even greater acidity as estimated from proton affinity values for the benzoic acid site relative to that of the phenol site was predicted, and accordingly, only a single isomer was sampled by TWIM-MS. Rationales for these contrasting trends based on interfering inductive effects, charge delocalization by resonance, and intraionic H-bonds that govern the acidity of benzoic acid relative to that of phenols are presented.