ENANTIOMER-SPECIFIC OXYGEN-EXCHANGE REACTIONS .2. ACID-CATALYZED WATER EXCHANGE WITH 1-PHENYL-1-ALKANOLS

The rate constants for three competing processes at the chiral center in the acid-catalyzed racemization of (R)-1-phenyl-1-propanol and (R)- 1-phenyl-1-butanol at 64.5 +/- 1.0 OC have been determined by chiral H PLC and GC/MS methods: oxygen exchange without inversion, k(E), oxygen exchange with inversion, k(EI), and inversion without exchange, k(I). These same rate constants, previously determined for natural abundanc e 1-phenylethanol in 50% O-18-enriched water, have been reevaluated fo r this compound by following the kinetics of 91% O-18-enriched alcohol in natural abundance water. These latter data strengthen the evidence that, for 1-phenylethanol, the departing water, in some cases, bonds to the opposite face of the intermediate carbocation as indicated by a non-zero value for k(I); this process is also operative in the reacti ons of the other two alkanols as shown by similar kinetic data. In ter ms of substitution reactions with the solvent leading to oxygen exchan ge, phenylpropanol behaves similarly to phenylethanol in that k(E) < k (EI); whereas, for phenylbutanol, k(E) approximate to K-EI. A common m echanism in which the initially formed carbocation is present as a com plex with the departing water, an ion-molecule pair, can account for t he variations in the relative rate constants for the oxygen exchange r eactions of these three alcohols. The rate of motion of water molecule s within the solvation sphere of these intermediates relative to their exchange with the bulk solvent to form randomly solvated carbocations differs, depending on the substituent at the chiral center. The varia tions in the rate constants for the oxygen exchange reactions reflect these differences in water mobility.