Study of the kinetics of tert-butyldimethylsulfonium ion solvolysis with application to the development of solvent nucleophilicity scale
Mahmood, Wan Ahmad Kamil B.
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The N̲[sub KL] scale of solvent nucleophilicity introduced by Kevill and Lin, based on the solvolysis of triethyloxo- nium (Et₃O⁺) ion, was established using the modified form of the extended (four-parameter) Grunwald-Winstein equation: N̲[sub KL] = log (k/kₒ)[sub Et₃O⁺] - 0.55Y⁺ where k is the solvolysis rate constant for any substrate reacting in S[sub N]1 mechanisms in any solvent relative to its rate, kₒ, in 80% aqueous-ethanol as the standard solvent. The Y⁺ values-modified solvent influence on leaving group parameters for charged substrates were obtained from a study of the solvolysis of t-butyldimethylsulfonium (t-BuMe₂S⁺) ions. Although some of the Y⁺ values could be obtained from previous experimental data, other Y⁺ values were estimated from: Y⁺ -0.09Y. The proportionality factor used was obtained from slope of log (k/kₒ)[sub t-BuMe₂S⁺] vs. the Y[sub t-BuCl] values. In view of the irregularity demonstrated by acetic acid in this plot, the present study was carried out to determine the Y⁺ values for aqueous-dioxane, aqueous-2,2,2-trifluoroethanol (TFE), acetic acid, ethanol (EtOH)-2,2,2-trifluoroethanol, formic acid, isopropyl alcohol and tert-butyl alcohol. These values were then used to recalculate new N̲[sub KL] values. It was also necessary to carry out solvolysis for triethyloxonium ions in ethanol-2,2,2-trifluoroethanol mixed solvent system, for no such studies were carried out before. The first order rate constants for the solvolysis of various t-butyldimethylsulfonium salts were found to be essentially independent of the anion. Using the trifluorome-thanesulfonate salt as the standard, in 80% ethanol, the iodide reacted 19% faster and for antimonate, it is 29% faster. From the solvolysis of t-butyldimethylsulfonium ions, m̲ values for various solvents and mixed solvents can be calculated from the plot of log (k/kₒ)[sub t-BuMe₂S⁺] vs. the Y values based on t-butyl chloride (t-BuCl) solvolysis. For a range of 90% dioxane to 20% dioxane (aqueous-dioxane) m̲ = -0.039, 100% 2,2,2-trifluoroethanol to 20% 2,2,2-trifluoroethanol (in aqueous mixture) m̲ = 0.220 and for 100% 2,2,2-trifluoroethanol to 100% ethanol (in TFE-EtOH mixed solvent) m̲ = -0.250. The N[sub KL] obtained for R-X⁺ systems in this work were then compared to N values for R-X systems, and are found to be in fairly good agreement.