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 (R-22 of 0.9960.999) 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.