New Insight into the Formation Mechanism of Imidazolium-Based Ionic Liquids from <i>N</i>‑Alkyl Imidazoles and Halogenated Hydrocarbons: A Polar Microenvironment Induced and Autopromoted Process

To illustrate the formation mechanism of imidazolium-based ionic liquids (ILs) from <i>N</i>-alkyl imidazoles and halogenated hydrocarbons, density functional theory calculations have been carried out on a representative system, the reaction of <i>N</i>-methyl imidazole with chloroethane to form 1-ethyl-3-methyl imidazolium chloride ([Emim]­Cl) IL. The reaction is shown to proceed via an S_N2 transition state with a free energy barrier of 34.4 kcal/mol in the gas phase and 27.6 kcal/mol in toluene solvent. The reaction can be remarkably promoted by the presence of ionic products and water molecules. The calculated barriers in toluene are 22.0, 21.7, and 19.9 kcal/mol in the presence of 1–3 ionic pairs of [Emim]­Cl and 23.5, 21.3, and 19.4 kcal/mol in the presence of 1–3 water molecules, respectively. These ionic pairs and water molecules do not participate directly in the reaction but provide a polar environment that favors stabilizing the transition state with large charge separation. Hence, we propose that the synthesis of imidazolium-based ILs from <i>N</i>-alkyl imidazoles and halogenated hydrocarbons is an autopromoted process and a polar microenvironment induced reaction, and the existence of water molecules (a highly polar solvent) in the reaction may be mainly responsible for the initiation of reaction