Regulating the Acidity of SO₃ H-Functionalized Ionic Liquids: Hydrogen Bonding or Electrostatic Potential?

The SO₃ H-functionalized ionic liquids (ILs) with high acidity are important catalysts for acid-catalyzed reactions. However, the acidity of these ILs have been found to decrease due to intramolecular hydrogen bonds (H-bonds). In this work, a series of anionic SO₃ H-functionalized ILs were designed and the factors resulting in weak acidity were investigated, including H-bonds strength and electrostatic potential on the leaving proton (ESPLP). Using catalytic experiment, atoms in molecules topology analysis and electrostatic potential calculation, it was found that the acidity of ILs was correlated with the value of ESPLP rather than the H-bond strength. Meanwhile, there were several ways to increase the acidity of anionic SO₃ H-functionalized ILs, such as enhancing the electron-withdrawing ability of cation, introducing H-bond acceptor sites on cation or separating the intramolecular H-bonds through substitution position. These strategies made the conversion of TBA in Friedel-Crafts alkylation increase from 19 % to 84 %.