α-, β-, and γ-Deuterium isotope effects in the solvolysis of octan-2-yl p-bromobenzenesulphonate in 88% aqueous trifluoroethanol and 83% aqueous hexafluoroisopropyl alcohol

Abstract

Solvolysis of octan-2-yl p-bromobenzenesulphonate (1) and its α-²H₁, β-²H₃, β-²H₂{[3,3-²H₂]-(1)}, and γ-²H₂ deuteriated derivatives in 88% w/w aqueous trifluoroethanol and 83% w/w aqueous hexafluoroisopropyl alcohol at 66 °C gave isotope effects k_H/k_D of 1.154, 1.250, 1.503, and 0.997 and 1.174, 1.270, 1.553, and 1.000, respectively. These results when supplemented with those earlier reported in 65% w/w aqueous ethanol represent a complete α- to γ-deuterium isotope effect spectrum in the solvolysis of a model secondary straight-chain alkyl system in aqueous hexafluoroisopropyl alcohol and a group of solvents covering a wide spectrum of nucleophilic ability and proposed as a model for the study of solvolytic reactions. The isotope effects increase in the order 65% w/w ethanol < 88% w/w trifluoroethanol < 83% w/w hexafluoroisopropyl, alcohol, corresponding to the decrease in the nucleophilic properties of these media. These trends are discussed in terms of Winstein's mechanistic scheme on one hand, and some features of our unified mechanism on the other.