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Kinetics and mechanism of the hydrolysis of substituted phenyl benzoates catalyzed by the o ‐iodosobenzoate anion
Author(s) -
El Seoud Omar A.,
Martins Monica F.
Publication year - 1995
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.610081003
Subject(s) - chemistry , nucleophile , catalysis , steric effects , medicinal chemistry , hydrolysis , tetrahedral carbonyl addition compound , reaction rate constant , leaving group , kinetic isotope effect , kinetics , stereochemistry , organic chemistry , physics , deuterium , quantum mechanics
The o ‐iodosobenzoate (IBA − )‐catalyzed hydrolysis of the following esters was studied spectrophotometrically: 4‐nitrophenyl 4‐X‐benzoate (series I, a–e, X = CH 3 , H, Cl, CN, NO 2 ), 4‐Y‐phenyl 4‐nitrobenzoate (series II, a–d, Y = CH 3 , H, Cl, CN), 2,4‐dinitrophenyl 4‐X‐benzoate (series III, b–e, X = H, Cl, CN, NO 2 ) and 4‐Y‐phenyl 2,4‐dinitrobenzoate (series IV, a–e, Y = CH 3 , H, Cl, CN, NO 2 ). Direct detection of a reaction intermediate, viz 1‐(4‐nitrobenzoyloxy)‐1,2‐benzodoxol‐3(1 H )‐one, the absence of (general base) catalysis by the leaving group, the small kinetic solvent isotope effect and the fast catalyst ‘turnover’ show that the mechanism of catalysis by IBA − is nucleophilic. Catalytic rate constants, activation parameters and application of the Hammett equation show the following mechanistic features: the first step of the reaction, i.e. that leading to the formation of the above‐mentioned intermediate is irreversible, the leaving group is the phenol and the rate‐limiting step is the collapse of the tetrahedral intermediate that is formed by attack of IBA − on the ester. A comparison of the results of hydrolysis by IBA − and by imidazole (Imz) revealed that IBA − and Imz differ in their sensitivity toward an ester structure and, for the same ester series, IBA − is a much more efficient catalyst. The reasons for the higher catalytic activity of IBA − include nucleophilicity enhancement due to the α‐effect, a smaller influence of steric effects and a more favorable entropy of activation.