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Intramolecular catalysis and the rate‐determining step in the alkaline hydrolysis of ethyl salicylate
Author(s) -
Khan M. Niyaz,
Gambo S. K.
Publication year - 1985
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550170408
Subject(s) - chemistry , reaction rate constant , intramolecular force , catalysis , alkoxide , hydrolysis , alkaline hydrolysis , hydroxide , entropy of activation , aqueous solution , ketene , ion , medicinal chemistry , rate determining step , kinetics , inorganic chemistry , stereochemistry , organic chemistry , physics , quantum mechanics
The alkaline hydrolysis of ethyl salicylate has been studied at 35°C within the [ŌH] range of 0.001–2.00 M. The observed hydroxide ion concentration dependence of rate has been explained by proposing the occurrence of two parallel kinetic steps shown as in the rate law: rate = k 1 [H 2 O] [E S ] + k 2 [ŌH] [E S ] where ES − represents ionized ethyl salicylate. The value of k 1 , is ca. 10 6 times larger than the expected value of rate constant for uncatalyzed aqueous cleavage of ethyl‐ p ‐hydroxybenzoate. This rate advantage is attributed to intramolecular general base catalysis. The analysis of observed activation parameters indicates that ca. 10 6 times rate enhancement is entirely due to favorable entropy change. The Brønsted‐type plots show an extremely low sensitivity of rate constants k 1 and k 2 with respective Brønsted coefficient of β lg 1= −0.03 ± 0.01 and β lg 2= −0.01 ± 0.05, on the basicity of leaving groups of salicylate esters (alkoxide and phenoxide ions). The low values of these Brønsted coefficients indicate essentially little or an insignificant amount of bond cleavage between carbonyl carbon and leaving group in the rate‐determining step in both the k 1 and k 2 steps. The rate constants obtained at different ethanol concentration follow Grunwald‐Winstein mY equation with m = 0.14 ± 0.01.