Kinetics of Decarboxylation of Pentafluorobenzoic Acid and Quinolinic Acid in High-Temperature Water

Decarboxylation of aromatic acids in high-temperature water has been studied previously because of its importance in green chemical synthesis, fuel processing, and geochemistry. Benzoic acid (or its derivatives) is typically used as a model compound. One expects electron-withdrawing substituents to lower the thermal stability of benzoic acid and make decarboxylation easier. To further advance understanding of the effect of electron-withdrawing substituents and the aromatic ring composition on the decarboxylation, two compounds were selected for study. Pentafluorobenzoic acid, with five fluorine substituents on the benzoic acid, can be used to examine the effect of electron-withdrawing groups. Quinolinic acid, which is the N-heterocyclic analog of terephthalic acid, can be used to examine the effect of a heterocyclic N atom and decarboxylation of a di-acid. Moreover, both compounds are important intermediates in the chemical or pharmaceutical industry. We conducted experiments at different reactant initial concentrations, different batch holding times, and different temperatures to fully characterize the reactivity of these two aromatic acids in high temperature water. A first-order rate law adequately described the data as shown by the agreement between the model (solid lines) and experimental results (discrete points) in the figures below. The activation energies were determined to be around 150 kJ•mol-1 for both compounds. In addition to elucidating the reaction kinetics, we will discuss the effect of electron-withdrawing substituents on the decarboxylation of benzoic acids.