A MECHANISM FOR CARBON ISOTOPE EXCHANGE BETWEEN AQUEOUS ACETIC ACID AND CO₂/ HCO₃^-: AN AB INITIO STUDY

The aqueous exchange of carbon atoms between organic acids, such as acetic acid, and dissolved inorganic carbon is an important topic in the chemistry of oil field waters. The exchange of carboxylic carbon atoms between acetic acid/acetate and CO₂/HCO₃^- may conceivably pass through a malonic acid intermediate or a hydroxylated malonic acid intermediate. Model reactions to and from these intermediates were investigated using molecular orbital and transition state theory (MO-TST) modeling at the B3LYP/6-31+G(d,p) level. Zero-point energy corrected barriers (ZPECB) and solvation corrected Gibbs free energies of activation predict that the slow steps are the isomerization reactions from acetic acid and acetate to CH₂=C(OH)₂ and CH₂=C(OH)O^-, respectively. Using these reactions to represent the acid and basic conditions, rate constants were computed. The involvement of water molecules in the transport of protons significantly lowers the ZPECB of the slow steps. Rate constants from 298K to 673K for the basic reaction are about 1 order of magnitude higher than rate constants of the acidic reaction and therefore indicate that the reaction under basic pH conditions should be faster than at acidic pH assuming equal concentrations of acetic acid and acetate. Regression analyses yield activation energies of 176 kJ/mol for the acidic case and 165 kJ/mol for the basic case. For an aqueous system of acetic acid or acetate and dissolved inorganic carbon at or above 473K, the results suggest that malonic acid may be present and isolable because malonic acid is a reaction intermediate in a relatively deep potential energy well.