Kinetics and mechanism of the hydrolysis of substituted phenyl benzoates catalyzed by theo-iodosobenzoate anion

Autor: Omar A. El Seoud, Monica F. Martins
Rok vydání: 1995
Předmět:
Zdroj: Journal of Physical Organic Chemistry. 8:637-646
ISSN: 1099-1395
0894-3230
Popis: The o-iodosobenzoate (IBA−)-catalyzed hydrolysis of the following esters was studied spectrophotometrically: 4-nitrophenyl 4-X-benzoate (series I, a–e, X = CH3, H, Cl, CN, NO2), 4-Y-phenyl 4-nitrobenzoate (series II, a–d, Y = CH3, H, Cl, CN), 2,4-dinitrophenyl 4-X-benzoate (series III, b–e, X = H, Cl, CN, NO2) and 4-Y-phenyl 2,4-dinitrobenzoate (series IV, a–e, Y = CH3, H, Cl, CN, NO2). Direct detection of a reaction intermediate, viz 1-(4-nitrobenzoyloxy)-1,2-benzodoxol-3(1H)-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.
Databáze: OpenAIRE
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