Mechanisms of decarboxylation of phenylacetic acids and their sodium salts in water at high temperature and pressure

Mechanistic interpretations of how organic reactions happen can enhance the predictive capacity of models for geochemical transformations built on thermodynamic and kinetic relations. In this study, the mechanisms of hydrothermal carboxylic acid decarboxylation reactions are explored using aqueous solutions of the model compound phenylacetic acid, its ring-substituted derivatives, and their sodium salts. Time-series experiments in gold capsules are performed at 300 °C and 1034 bar, and analyzed using gas chromatography. The decarboxylation products are the appropriately substituted toluene and CO₂ or HCO₃ ⁻ depending on the pH. It is found that the decarboxylation reaction is irreversible at the studied conditions, consistent with the expectation of a strong thermodynamic drive at elevated temperatures. Decarboxylation of both the acid and anion forms of phenylacetic acid follow first-order kinetics, with apparent rate constants of 0.044 ± 0.005 h⁻¹ ([1.2 ± 0.14] × 10⁻⁵ s⁻¹) and 0.145 ± 0.02 h⁻¹ ([4.0 ± 0.56] × 10⁻⁵ s⁻¹), respectively. However, trends in the reaction rate with changes in the electronic properties of methyl and fluoro substituents reveal that the two forms of phenylacetic acid decarboxylate via two different mechanisms. It is inferred that the associated phenylacetic acid molecule decarboxylates by the formation of a ring-protonated zwitterion, whereas the acid anion directly decarboxylates to a benzyl anion. Zwitterionic mechanisms of activation to decarboxylation may be applicable to other aromatic acids (e.g., benzoic acid) having unsaturation that is alpha- or beta- to the carboxyl group, as well as to aliphatic acids that can be converted to α,β- or β,γ-unsaturated acids in natural hydrothermal systems. For acids lacking suitable unsaturation (e.g., acetic acid) or at higher pH, carbanion mechanisms may predominate in hydrothermal fluids. In these cases, there is potential to predict decarboxylation rates from the pK_a of the hydrocarbon product. The finding of speciation-dependent reaction mechanisms implies that host rocks of aqueous fluids are key to determining decarboxylation rates, as a consequence of water-rock reactions that control fluid pH.