Reaction pathways and energy barriers for alkaline hydrolysis of carboxylic acid esters in water studied by a hybrid supermolecule-polarizable continuum approach

Reaction pathways, solvent effects, and energy barriers have been determined for the base-catalyzed hydrolysis of two representative alkyl esters in aqueous solution, using a hybrid supermolecule-polarizable continuum approach. Four solvent water molecules were explicitly included in the supermolecular reaction coordinate calculations; the remaining solvent water was modeled as a polarizable dielectric continuum surrounding the supermolecular reaction system. Two competing reaction pathways were observed, sharing a common first step, i.e. the formation of the tetrahedral intermediate. One pathway involves a direct proton transfer in the second step, i.e. the decomposition of the tetrahedral intermediate. A second pathway involves a water-assisted proton transfer during the decomposition of the tetrahedral intermediate. The direct participation of the solvent water molecule in the proton-transfer process significantly drops the energy barrier for the decomposition of the tetrahedral intermediate. Thus, the energy barrier calculated for the decomposition of the tetrahedral intermediate through the water-assisted proton transfer becomes lower than the barrier for the formation of the tetrahedral intermediate, while that through the direct proton transfer is higher. The computations reveal the important effect of solvent hydrogen bonding on energy barriers; without explicit consideration of the hydrogen-bonding effects, the calculated energy barriers for the formation of the tetrahedral intermediate become ~,4-5 kcal/mol smaller. The favorable pathway involving water-assisted proton transfer and the energy barriers calculated using the hybrid supermolecule- polarizable continuum approach, including both the hydrogen-bonding effects and the remaining bulk solvent effects, are consistent with available experimental results. The energy barriers calculated for the first step of the hydrolysis in aqueous solution are in excellent agreement with the reported experimental data for methyl acetate and methyl formate.