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

Abstract

The sum of the rate constants for solvolysis and scrambling of carbon bridging and nonbridging oxygen-18 at 4-MeC(6)H(4)CH(CF(3))OS((18)O(2))Tos in 50/50 (v/v) trifluoroethanol/water, (k(solv) + k(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 × 10(10) 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 × 10(9) 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-MeC(6)H(4)CH(CH(3))O(2)CC(6)F(5)) 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.

Figure 1

Partial ¹³C spectra of unreacted substrate in C₆D₆ recovered during the solvolysis of 4-OS(¹⁸O)₂C₆H₄-4-Me in 50/50 (v/v) trifluoroethanol/water (I = 0.5, NaClO₄) at 25° C. The signal for the benzylic carbon is split by the α-fluorines (J = 34 Hz). (A) Initial spectrum of substrate 4-OS(¹⁸O)₂C₆H₄-4-Me. (B) Spectrum of substrate recovered after a 36 hour reaction time (3220 transients). (C) Spectrum of substrate recovered after a 48 hour reaction time (3560 transients). (D) Spectrum of substrate recovered after a 69 hour reaction time (10590 transients). The origin of the small peak at ca 78.6 ppm observed in Figures 1C and 1D is not known.

Figure 2

Time course for the formation of isomerized reaction product 4-¹⁸OS(¹⁸O,¹⁶O)C₆H₄-4-Me during solvolysis of 4-OS(¹⁸O)₂C₆H₄-4-Me in 50/50 (v/v) TFE/H₂O at 25 °C. The solid line shows the nonlinear least-squares fit of the data to eq 1 derived for Scheme 1 using k_solv = 3.6 × 10⁻⁶ s⁻¹ and k_iso = 1.8 × 10⁻⁶ s⁻¹. The upper and lower dashed lines show the fits obtained using values of k_iso that are 10% higher and 10% lower, respectively, than the value from least-squares analysis.

Scheme 1

Scheme 2

Scheme 3

Scheme 4