title: New Insight into the Formation Mechanism of Imidazolium-Based Ionic Liquids from N-Alkyl Imidazoles and Halogenated Hydrocarbons: A Polar Microenvironment Induced and Autopromoted Process.
abstract: To illustrate the formation mechanism of imidazolium-based ionic liquids (ILs) from N-alkyl imidazoles and halogenated hydrocarbons, density functional theory calculations have been carried out on a representative system, the reaction of N-methyl imidazole with chloroethane to form 1-ethyl-3-methyl imidazolium chloride ([Emim]Cl) IL. The reaction is shown to proceed via an SN2 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 N-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.
authors.affiliation: Institute of Theoretical Chemistry, School of Chemistry and Chemical Engineering, Shandong University , Jinan 250100, China.
authors.forename: Xueli
authors.identifier: http://orcid.org/0000-0003-3286-3786
authors.initials: X
authors.lastname: Mu
authors.affiliation: Dongguan Institute of Advanced Technology , Dongguan 523808, China.
authors.forename: Nan
authors.initials: N
authors.lastname: Jiang
authors.affiliation: Institute of Theoretical Chemistry, School of Chemistry and Chemical Engineering, Shandong University , Jinan 250100, China.
authors.forename: Chengbu
authors.initials: C
authors.lastname: Liu
authors.affiliation: Institute of Theoretical Chemistry, School of Chemistry and Chemical Engineering, Shandong University , Jinan 250100, China.
authors.forename: Dongju
authors.initials: D
authors.lastname: Zhang