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Dynamics of reaction of ion pairs in aqueous solution: racemization of the chiral ion pair intermediate of solvolysis of ( S )‐1‐(4‐methylphenyl)ethylpentafluorobenzoate
Author(s) -
Tsuji Yutaka,
Mori Tetsuo,
Toteva Maria M.,
Richard John P.
Publication year - 2003
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.611
Subject(s) - solvolysis , chemistry , racemization , reaction rate constant , aqueous solution , ion , carbocation , solvent , medicinal chemistry , stereochemistry , crystallography , organic chemistry , kinetics , hydrolysis , physics , quantum mechanics
The chiral ester( S )‐1‐(4‐methylphenyl)ethyl pentafluorobenzoate, ( S )‐1‐OC(O)C 6 F 5 , 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)C 6 F 5 in 50:50 (v/v) trifluoroethanol–water ( I = 0.50, NaClO 4 ). 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 6 F 5 , 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 6 F 5 . 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 × 10 10 s −1 for inversion of the chiral ion pair, which is similar to k −d = 1.6 × 10 10 s −1 for its separation to free ions. Copyright © 2003 John Wiley & Sons, Ltd.