Abstract
The kinetics of the hydrolysis of benzyl hydrogen phthalate 1 was studied at OH- concentration within 0.05 and 0.5 M. The reaction does not manifest appreciable intramolecular catalysis. The base catalyzed rate constant is about two orders of magnitude faster than the value predicted by the Bronsted plot defined by aryl hydrogen phthalate esters. In the presence of hydroxypropyl-βcyclodextrin (HPCD) at NaOH = 0.45 M and at pH= 12.8 the observed rate constant for the hydrolysis decreases as the HPCD concentration increases in a non-linear fashion. The kinetic results are interpreted in terms of the formation of an inclusion complex of the ionized substrate KCD A , with HPCD. In this complex the carboxylate group forms hydrogen bonds with the hydroxyl groups at the rim of the cyclodextrin cavity and the ionised group of the cyclodextrin is kept away from the reaction center due to electrostatic repulsion.
Highlights
The study of intramolecular catalysis in model systems gives important information about enzyme mechanisms.[1]
The addition of cyclodextrin significantly decreases the rate of hydrolysis due to the formation of an inclusion compound with an unfavourable structure for the intramolecular reaction of the substrate with the ionized hydroxyl group of cyclodextrin
In order to establish if this is the reason for the inhibition observed in the reaction of substrate 1, we determined the effect of HPCD on the basic hydrolysis of p-chlorophenyl hydrogen phthalate, which has a much better leaving group than 1
Summary
The study of intramolecular catalysis in model systems gives important information about enzyme mechanisms.[1]. The addition of cyclodextrin significantly decreases the rate of hydrolysis due to the formation of an inclusion compound with an unfavourable structure for the intramolecular reaction of the substrate with the ionized hydroxyl group of cyclodextrin.
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