Gas-Phase Acidities of α- and α,α-SO2CF3-Substituted Toluenes. Varying Resonance Demand in the Electron-Rich System

The gas-phase acidities (GA) of aryl(trifluoromethylsulfonyl)methanes (ArCH2SO2CF3; 1) and arylbis(trifluoromethylsulfonyl)methanes (ArCH(SO2CF3)(2); 2) were determined by measuring proton-transfer equilibria. Substituent effects for acidities of a series of ArCH(R-1)R-2 including 1 and 2 have been analyzed successfully in terms of the Yukawa-Tsuno equation. The resonance demand parameter r(-) value was found to decrease linearly with increasing acidity of the GA values of the unsubstituted parent carbon acids, and the change of the r(-) value was correlated with the acidifying effect of the phenyl group (R = Ph) in the RCH2(R-1)R-2. In addition, the geometric features and natural charges of the conjugate anions calculated at B3LYP/6-311+G(d,p) were found to be correlated linearly with the r(-) values. Such behavior of the resonance demand parameter in the electron-rich system, ArC-(R-1)R-2, is completely consistent with that observed for the electron-deficient system, ArC+(R-1)R-2, revealing that the resonance demand is contingent upon the structure of carbanions and carbocations. Furthermore, it was found that the rho values also decreased with increasing acidity of the GA values of the unsubstituted parent carbon acids. This would be related to the distribution of the charge between the aromatic moiety and the C(R-1)R-2 moiety.