Reaction mechanism study for the synthesis of alkylimidazolium-based halide ionic liquids

The reaction mechanisms of 1-methylimidazole with a number of alkyl halides including ethyl chloride, butyl chloride, ethyl bromide and butyl bromide were determined using density-functional theory(DFT). At the B3LYP/ 6-31++G**//B3LYP/6-31G* basis sets, two mechanisms were found: path A (reactants -> TS1 -> P1) and path B (reactants -> TS2 -> P2). For path A, the halide ions form hydrogen bonds with the C2-H; for path B, the halide ions form hydrogen bonds with the C5-H. Hydrogen bonds play a fundamental role in the synthesis of the alkylimidazolium-based ionic liquids, especially the hydrogen on the C2 affects the reaction significantly. The activation energies of reactions passing through TS1 are lower than that passing through TS2, which indicate that reactions passing through TS1 are favored by reaction dynamics. In agreement with experimental findings, the calculations showed that the reactions through TS1 are exothermic processes.