Computational study on the hydrolysis of halomethanes

Abstract

The hydrolysis reactions of methane and halomethanes have been systematically investigated using density functional theory. These reactions may occur via a metathesis mechanism and a direct-elimination mechanism, metathesis being the predominant pathway with the formation of methanol and HX (X = H, F, Cl, Br, I). The energy barriers of the predominant gas-phase hydrolysis pathway for CH4, CH3F, CH3Cl, CH3Br, and CH3I are 92.9, 19.3, 25.1, 18.8, and 20.8 kcal/mol, respectively. These values change to 92.6, 20.0, 22.8, 16.3, and 18.3 kcal/mol, respectively, when polarizable continuum model implicit solvent effect is employed. The catalytic influence of molecular water and the bulk solvent effect of water have been revealed. It was also determined that the energy barrier initially decreased and then increased with an increase in the number of water molecules. For CH4 and CH3F, there are four reactive H2O molecules taking part in the proton transfer during hydrolysis to form ten-member rings in the transition state, and two other H2O molecules participate as solvents in the predominant route. For CH3Cl, CH3Br, and CH3I, there are three H2O molecules that form eight-member rings transition state and other 2H2O molecules that act as solvent in the predominant hydrolysis pathway. Additional H2O molecules as an explicit solvent are investigated using the ONIOM model, and the coulomb interaction between adjacent atoms of the transition state is calculated to investigate the inherent reason for the formation of HF in the gas phase and (H3O+ + F−) in a solvent. These intrinsic mechanistic insights should facilitate a deeper understanding of halomethanes hydrolysis.