Kinetics and solvent effects in the synthesis of ionic liquids: imidazolium

Abstract

Ionic liquids (ILs) are being considered as a promising class of potentially environmentally-friendly (“green”) solvents and materials for use in a variety of applications. However, ionic liquids are conventionally synthesized by batch, without known kinetics, in non-sustainable solvents. For ILs to be a truly “green” technology for widespread use, they must themselves be made in a correspondingly benign manner for low cost, as enabled by process development. This investigation will illustrate the kinetics and large solvent effects in the synthesis of 1-hexyl-3-methyl-imidazolium bromide in 10 different solvents: acetone, acetonitrile, 2-butanone, chlorobenzene, dichloromethane, dimethyl sulfoxide (DMSO), ethyl formate, ethyl lactate, methanol, and cyclopentanone. The kinetic rate constant for the synthesis in DMSO is over an order-of-magnitude larger than that in methanol. While the kinetic rate of these type of SN2 reactions is generally known to increase with solvent “polarity”, multi-parameter solvent descriptors, e.g. of Kamlet and Taft, are required to quantify these effects in a Linear Solvation Energy Relationship. These relationships are used with environmental and toxicity databases, such as the Rowan Solvent Selection Table, to rapidly optimize the solvent for favorable kinetics and minimal human and environmental impact.