Double-Line Hammett Relationship Revealed through Precise Acidity Measurement of Benzenethiols in Neat Ionic Media: A Typical “Ionic Liquid Effect”?

Equilibrium acidities (pK_a) of 14 aromatic thiols in four pure room temperature ionic liquids (RTILs) were precisely determined and the corresponding acidity scales were established for the first time. Regression analyses show a distinct double-line Hammett relationship with similar slopes and excellent linearity (R² of 0.996–0.999) in all four ILs. This could be rationalized by a resonance enhanced effect of the IL cation to solvate the para substituent of feasible electronic structure (CSAR effect), revealing a typical and rarely seen “ionic liquid effect”.