The Effects of α-Substituents on the Kinetic and Thermodynamic Stability of 4-Methoxybenzyl Carbocations: Carbocation Lifetimes That Are Independent of Their Thermodynamic Stability

The following new rate constants for reaction of α-substituted 4-methoxybenzyl carbocations, 4-MeOC₆H₄CR¹(R²)⁺, with a solvent of 50:50 (v/v) trifluoroethanol/water at 25 °C and ionic strength 0.50 (NaClO₄) are reported: 4-MeOC₆H₄CH(OMe)⁺, k_s = 2.2 × 10⁷ s⁻¹; 4-MeOC₆H₄CH(N₃)⁺, k_s = 3.3 × 10⁵ s⁻¹; 4-MeOC₆H₄C(CH₃)₂⁺, k_s = 1.3 × 10⁷ s⁻¹; 4-MeOC₆H₄CH(CO₂Et)⁺, k_s = 1.4 × 10⁷ s⁻¹; 4-MeOC₆H₄CCH₃(CF₃)⁺, k_s = 2.5 × 10⁷ s⁻¹. The values of k_s for reaction of 4-MeOC₆H₄CR¹(R²)⁺ with 50:50 (v/v) trifluoroethanol/water are nearly independent of very large changes in the thermodynamic stability of these carbocations caused by the addition of a wide range of electron-withdrawing or electron-donating groups at the α-position. In the most extreme case, the change from an α-methoxy to two α-(trifluoromethyl) substituents leads to a 23 kcal/mol thermodynamic destabilization of 4-MeOC₆H₄CR¹(R²)⁺ relative to the neutral azide ion adducts but a 5-fold decrease in its reactivity toward solvent. The data show that the effects of α-substituents on the kinetic stability of 4-MeOC₆H₄CR¹(R²)⁺ are complex and do not parallel the thermodynamic stability of these carbocations. The results are explained by consideration of the polar and resonance effects of the α-substituents on both the thermodynamic driving force and the intrinsic barrier for capture of the carbocations by solvent. These reactions are a new example of the consequences of the “principle of nonperfect synchronization”.