Straightforward synthesis of the Brønsted acid hfipOSO3H and its application for the synthesis of protic ionic liquids.

Abstract

The easily accessible hexafluoroisopropoxysulfuric acid (1, hfipOSO3H; hfip = C(H)(CF3)2) was synthesized by the reaction of hexafluoroisopropanol and chlorosulfonic acid on the kilogram scale and isolated in 98 % yield. The calculated gas-phase acidity (GA) value of 1 is 58 kJ mol(-1) lower in ΔG° than that of sulfuric acid (GA value determined by a CCSD(T)-MP2 compound method). Considering the gas-phase dissociation constant as a measure for the intrinsic molecular acid strength, a hfipOSO3H molecule is more than ten orders of magnitude more acidic than a H2SO4 molecule. The acid is a liquid at room temperature, distillable at reduced pressure, stable for more than one year in a closed vessel, reactive towards common solvents, and decomposes above 180 °C. It is a versatile compound for further applications, such as the synthesis of ammonium- and imidazolium-based air- and moisture-stable protic ionic liquids (pILs). Among the six synthesized ionic compounds, five are pILs with melting points below 100 °C and three of them are liquids at nearly room temperature. The conductivities and viscosities of two representative ILs were investigated in terms of Walden plots, and the pILs were found to be little associated ILs, comparable to conventional aprotic ILs.