Formation and Stability of Ring-Substituted 1-Phenylethyl Carbocations¹

The solvolysis of 1-phenylethyl derivatives with electron-donating 4-substituents in 50:50 trifluoroethanohwater (v:v) occurs at a rate that is independent of azide concentration but gives yields of the corresponding azide adducts of up to 98% by trapping a carbocation intermediate. Rate constants for reactions of the cations with solvent range from 2 × 10³ s⁻¹ (4-Me₂N) to 4 × 10⁹ s⁻¹ (4-Me), assuming a diffusion-controlled rate constant of 5 × 10⁹ M⁻¹ s⁻¹ for their reactions with azide and thiol anions. Correlation of the rate constants following the Yukawa-Tsuno treatment gives pⁿ = 2.5, p^r = 5.2, and r⁺ = 2.1 for the reaction with trifluoroethanol, and pⁿ = 2.7, p^r = 4.9, and r⁺ = 1.8 for the reaction with water. The reverse reaction, acid-catalyzed cleavage of substituted 1-phenylethyl alcohols to give the corresponding carbocation, follows pⁿ = −4.9_; p^r = −4.4, and r⁺ = 0.9. This gives values of pⁿ= −7.6, p^r = −9.3, and r⁺ = 1.2 for formation of the cations at equilibrium. There is an imbalance in the development of resonance delocalization, analogous to the “nitroalkane anomaly”, that is consistent with a dependence of the fraction of maximal resonance delocalization on the fraction of rehybridization or C-X bond cleavage. Solvent effects on carbocation stability in aqueous-organic mixtures are relatively small. They depend mainly on the nucleophilicity of the solvent components and a specific solvent effect of trifluoroethanol on the reactivity of hydroxyiic nucleophiles, including trifluoroethanol itself. The “ionizing power” of the solvent has only a small effect on cation stability, and there is little effect of the concentration or nature of added salts.