Dynamics for reactions of ion pairs in aqueous solution: Reactivity of tosylate anion ion paired with the highly destabilized 1-(4-methylphenyl)-2,2,2- trifluoroethyl carbocation

The sum of the rate constants for solvolysis and scrambling of carbon bridging and nonbridging oxygen-18 at 4-MeC₆H₄CH(CF ₃)OS(¹⁸O₂)Tos in 50/50 (v/v) trifluoroethanol/water, (k_solvRk_iso)=5.4 × 10 ^-6 s^-1, is 50% larger than k_solv=3.6 × 10^-6 for the simple solvolysis reaction of the sulfonate ester. This shows that the ion-pair intermediate of solvolysis undergoes significant internal return to form reactant. These data give a value of k_-1 =1.7 × 10¹⁰ s^-1 for internal return of the carbocation-anion pair to the substrate. This rate constant is larger than the value of k_-1=7 × 10⁹ s^-1 reported for internal return of an ion pair between the 1-(4-methylphenyl)ethyl carbocation and pentafluorobenzoate anion to the neutral ester (4-MeC₆H ₄CH(CH₃)O₂CC₆F₅) in the same solvent. The partitioning of ion pairs to the 1-(4-methylphenyl)ethyl carbocation and to the highly destabilized 1-(4-methylphenyl)2,2,2- trifluoroethyl carbocation is compared and contrasted.