The structure-activity relationship and physicochemical properties of acetamide-based brønsted acid ionic liquids

We prepared three series of novel protic ionic liquids (PILs) based on acetamide and a Brønsted acid (HX, where X is CF₃COO ^-, CH₃COO^-, or HSO₄^-) by a simple atom-economic neutralization reaction. We investigated their physicochemical properties, such as acidic scale, viscosity, ionic conductivity, and thermal characteristics, as well as the structure-activity relationships of these PILs. Carboxylic acid formation is only observed in the acetamide sulfate complex system by FT-IR. The ionic conductivities for most of the samples are between 10^-3 and 10^-1 S/m at room temperature and at low viscosity. Acetamide trifluoroacetate (ATFA) with a molar ratio of 5/5 (acetamide/trifluoroacetic acid) has an ionic conductivity of 0.25 S/m, and its viscosity is only 10 cP at 25 °C. The samples exhibit better properties at higher temperatures. ATFA with molar ratios of 7/3 and 8/2 have ionic conductivities of 1.13 and 1.07 S/m at 80 °C, respectively. Moreover, most of the prepared samples possess relatively moderate thermal stabilities (up to 106 °C for ATFA) and a wide liquid range (down to -69 °C for ATFA). All these properties make these acetamide-based PILs of interest as reaction media, as catalysts in organic synthesis, or as electrolytes in fuel cells.