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 18O-scrambling of 4-MeC6H4 13CH(Me)18OC(O)C6F5 in 50/50 (v/v) trifluoroethanol/water, k solv + k iso = 1.22 × 10-5 s-1, is larger than k solv = 1.06 × 10-5 s-1 for solvolysis of the unlabeled ester. This shows that the ion pair intermediate undergoes significant internal return. The data give k - 1 = 7 × 109 s-1 for internal return by unimolecular collapse of the ion pair, which is significantly larger than k Nu = 5 × 108 M-1 s-1 for bimolecular nucleophilic addition of carboxylate anions to 4-MeC6H4CH(Me)+.