Acidity of carboxylic acids: A rebuttal and redefinition

A recent theory of ionization of carboxylic acids divided the process of io nization into two steps and attributed the main importance to the electrost atic potential of the acid molecule. The origin of the acidity was thus see n in the high energy of the acid molecule and not in the stabilization of t he anion by resonance. In this paper, the theory was revised on the basis o f further calculations at an MP2/6-31++G(2d,p) level which followed in two steps the reverse process, protonation of the formate anion. The results we re compared to those of the methanolate anion as reference. A contradictory conclusion was obtained: the reason for the acidity is in the electrostati c potential of the anion. As a model compound without resonance, 2,2,2-trif luoroethanol was investigated in the same way. The results were qualitative ly similar, although any resonance in the anion is not possible. One can co nclude that the acidity of carboxylic acids is due to the low energy of the ir anions; this follows unambiguously from the isodesmic reactions on the b asis of either the experimental enthalpies of formation or the calculated e nergies. It is more difficult to decide whether this low energy is caused b y resonance, because any model of the carboxylate anion without resonance i s necessarily imperfect and the whole concept is not exactly defined. Sever al such models were reexamined and improved by separating the effect operat ive in the anion from those in the neutral acid molecule. While the electro static model did not allow any significant conclusion, two models based on VB calculation and on correlation analysis, respectively, furnished qualita tively concordant results: resonance in the anion is diminished by the reso nance in the acid molecule, and both together are responsible for less than one-half of the acidity enhancement as compared to the acidity of alcohols . The ratio is reversed in water solution: resonance is then the more impor tant factor responsible for some two-thirds of the enhanced acidity.