Reactions of ion-pair intermediates of solvolysis.

The detection during solvolysis of competing signature reactions such as the racemization of a chiral substrate, or the scrambling of oxygen isotopes at the leaving group serves as evidence for a stepwise mechanism that proceeds through a reversibly formed ion-pair intermediate. We have examined these signature reactions and determined the relative rates of competing solvolysis, isomerization, and racemization reactions of neutral ring-substituted 1-phenylethyl derivatives. These data were used to define the relative rate constants for partitioning of carbocation-anion pairs between dynamic processes such as reorganization of the ion pair within a solvent cage, and migration of the leaving group anion from the "top" to the "bottom" face of the planar carbocation. We present here a well-defined picture of the dynamics of these reactions in the mostly aqueous solvent of 50/50 (v/v) trifluoroethanol/water. A more complicated problem is the description of the changes in the reaction energy profiles for solvolysis, isomerization, and racemization reactions that occur as one destabilizes the carbocation intermediate of the stepwise reaction. We have examined the signature isomerization and racemization reactions of a ring-substituted 1-phenylethyl derivative that reacts by a "borderline" mechanism. We suggest that the signature isomerization reaction may in fact follow a concerted mechanism that bypasses the putative carbocation-anion pair intermediate.