Dynamics of reaction of ion pairs in aqueous solution: Racemization of the chiral ion pair intermediate of solvolysis of (S)-1-(4-methylphenyl)ethyl pentafluorobenzoate

The chiral ester (S)-1-(4-methylphenyl)ethyl pentafluorobenzoate, (S)-1-OC(O)C₆F₅, was prepared and a value of k _rac = 8.5 × 10^-7 s^-1 was determined for the rate constant for its racemization to (R)-1-OC(O)C₆F₅ in 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO₄). This rate constant is 12 times smaller than k_solv = 1.06 × 10 ^-5 s^-1 for the stepwise solvolysis of 1-OC(O)C ₆F₅, which shows that inversion of the ion-pair intermediate is a relatively rare event during solvolysis in a largely aqueous solvent, and two times smaller than k_iso = 1.6 × 10 ^-6 s^-1 for degenerate isomerization which exchanges the position of the ester bridging and non-bridging oxygens of 1-OC(O)C ₆F₅. A simple relationship is derived between the empirical rate constant ratio k_rac/k_iso and the microscopic rate constants for reaction of the ion-pair intermediate within its solvation shell. Substitution of estimated rate constants for reactions of the 1-(4-methylphenyl)ethyl carbocation-pentafluorobenzoate anion pair into this equation gives k_i = 1.5 × 10¹⁰ s^-1 for inversion of the chiral ion pair, which is similar to k_-d = 1.6 × 10¹⁰ s^-1 for its separation to free ions.