Oxygen exchange as a function of racemization in 1-phenyl-1-ethanol. Kinetic evidence for ion-dipole pair intermediates

The rate constants for three competing processes at the chiral center in the acid-catalyzed racemization of both (R)- and (S)-1-phenyl-1-ethanol in 50% 18O-enriched water at 64.5 °C have been determined by chiral HPLC and GC/MS methods: oxygen exchange without inversion, k(E) = (5.9 ± 0.6) x 10-4 M-1 s-1, inversion without oxygen exchange, k(I) = (4.1 ± 0.2) x 10-4 M-1 s-1, and oxygen exchange with inversion, k(EI) = (7.6 ± 0.7) x 10-4 M-1 s-1. The fact that k(I) is comparable to the other rate constants indicates that, in some cases, the departing water molecule bonds to the intermediate carbocation from the opposite face. The smaller value of k(E) relative to k(EI) indicates that the departing water shields the front side from attack by the bulk solvent. To account for the observed rate constants, a mechanism is proposed in which the initially formed carbocation intermediate is present as a tightly solvated ion-dipole with a finite lifetime. The water molecules within the solvation sphere exchange at a rate comparable to their exchange with the bulk solvent to form a randomly solvated carbocation. These proposed intermediates are analogous to the intimate and solvent separated ion pairs of solvolysis reactions.