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‐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. Copyright © 2010 John Wiley & Sons, Ltd.