Halogenation of imidazolium-based ionic liquids: Thermodynamic perspective

• Halogenation of the imidazolium-based cation was considered.
• Entropy, enthalpy and free energy for the halogenation reaction were derived using ab initio calculations.
• Fluorination is systematically more favorable than chlorination.
• The results encourage synthetic efforts to obtain the halogenated ionic liquids.

Imidazolium cations are promising for anion exchange membranes, electrochemical applications and gas capture. They can be chemically modified in many ways including halogenation. Halogenation possibilities of the imidazole ring constitute a particular interest. This work investigates fluorination and chlorination reactions of all symmetrically non-equivalent sites of the 1-ethyl-3-methylimidazolium cation. Halogenation of all carbon atoms is thermodynamically permitted. Out of these, the most favorable site is the first methylene group of the side alkyl chain. In turn, the least favorable site is carbon of the imidazole ring. Temperature dependence of enthalpy, entropy, and Gibbs free energy at 1 bar is discussed. The reported results provide an important guidance in functionalization of ionic liquids to foster search of task-specific compounds.

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