Dynamics of reaction of ion pairs in aqueous solution: racemization of the chiral ion pair intermediate of solvolysis of (S)‐1‐(4‐methylphenyl)ethylpentafluorobenzoate

Abstract

AbstractThe chiral ester (S)‐1‐(4‐methylphenyl)ethyl pentafluorobenzoate, (S)‐1‐OC(O)C6F5, was prepared and a value of krac = 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 ksolv = 1.06 × 10−5 s−1 for the stepwise solvolysis of 1‐OC(O)C6F5, 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 kiso = 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)C6F5. A simple relationship is derived between the empirical rate constant ratio krac/kiso 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 ki = 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. Copyright © 2003 John Wiley & Sons, Ltd.