Dynamics for reaction of an ion pair in aqueous solution: Reactivity of carboxylate anions in bimolecular carbocation-nucleophile addition and unimolecular ion pair collapse

The sum of the rate constants for solvolysis and O-18-scrambling of 4-(MeC6 H4CH)-C-13(Me)(OC)-O-18(O)C6F5 in 50/50 (v/v) triiluoroethanol/water, k(sol v) + k(iso) = 1.22 x 10(-5) s(-1), is larger than k(solv) = 1.06 x 10(-5) s (-1) for solvolysis of the unlabeled ester. This shows that the ion pair in termediate undergoes significant internal return. The data give k(-1) = 7 x 10(9) s(-1) for internal return by unimolecular collapse of the ion pair, which is significantly larger than k(Nu) = 5 x 10(8) M-1 s(-1) for bimolecu lar nucleophilic addition of carboxylate anions to 4-MeC6H4CH(Me)(+).