Gibbs energies of formation of chlorinated benzoic acids and benzoates and application to their reductive dechlorination

Abstract

Chlorinated benzoates are widely spread in the environment and are subject to anaerobic degradations, where chlorinated benzoates are used as electron acceptor by certain groups of anaerobic bacteria and the energy of the reduction reactions is used by bacteria for growth. The energies, namely the change of Gibbs energy of the reduction reaction (Δ_rG), are predicted here using quantum chemistry calculations, where the gas-phase properties, including the enthalpies of formation, the Gibbs free energies, and the acidities, are predicted at the G3XMP2 level, and the solvent effects are modelled using the polarizable conductor model (C-PCM) model. The predicted gas-phase enthalpies of formation and acidities of monochlorinated benzoic acids are in excellent agreement with the experimental measurements, and the aqueous phase pK_a prediction shows that chlorinated benzoic acids will exist as benzoates almost exclusively in the natural environment (pH ∼7).