Formation and Stability of Ring-Substituted 1-Phenylethyl Carbocations

Article abstract of DOI:10.1021/ja00317a031

The solvolysis of 1-phenylethyl derivatives with electron-donating 4-substituents in 50:50 trifluoroethanol:water(v:v) occurs at a rate that is independent of azide concentration but gives yields of the corresponding azide adducts of up to 98percent by trapping a carbocation intermediate.Rate constants for reactions of the cations with solvent range from 2 x 10³ s^-1 (4-Me2N) to 4 x 10⁹ s^-1 (4-Me), assuming a diffusion-controlled rate constant of 5 x 10⁹ M^-1 s^-1 for their reactions with azide and thiol anions.Correlation of the rate constants following the Yukawa-Tsuno treatment gives ρⁿ = 2.5, ρ^r = 5.2, and r⁺ = 2.1 for the reaction with trifluoroethanol, and ρⁿ = 2.7, ρ^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 ρⁿ = -4.9, ρ^r = -4.4, and r⁺ = 0.9.This gives values of ρⁿ = -7.6, ρ^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 hydroxylic 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.