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Kinetics and mechanism of the reaction of substituted O ‐benzoylbenzamidoximes with sodium methoxide in methanol
Author(s) -
Dušek Libor,
Kaválek Jaromír,
Štěrba Vojeslav
Publication year - 1999
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/(sici)1099-1395(199908)12:8<626::aid-poc173>3.0.co;2-w
Subject(s) - chemistry , sodium methoxide , reaction rate constant , methoxide , tetrahedral carbonyl addition compound , methanol , medicinal chemistry , reaction rate , sodium , kinetics , equilibrium constant , crown ether , catalysis , inorganic chemistry , ion , organic chemistry , nucleophile , physics , quantum mechanics
The kinetics of reaction of substituted O ‐benzoylbenzamidoximes with sodium methoxide in methanol were studied at 25 °C. The only reaction products are substituted benzamidoximes and methyl benzoates. The slope of the dependence of rate constant on sodium methoxide concentration gradually increases, but in the presence of C 18 crown ether the dependence becomes linear and the rate constant is lower than in the absence of the crown ether, which means that the reaction is catalysed by sodium cation. The rate constants of reactions with the ion pair and with methoxide ion were determined with the presumption that the rate‐limited step of the catalysed reaction is the reaction of substituted O ‐benzoylamidoximes with the ion pair of sodium methoxide. The rate constants of the reaction with the ion pair are about 20 times higher than those of the non‐catalysed process. The slopes of the dependence of log k of the non‐catalysed and catalysed reactions on the p K a of substituted benzamidoximes are 1.05 and 0.94, respectively. These high values indicate the rate‐limiting step involving the splitting off of substituted benzamidoxime from the tetrahedral intermediate. On the basis of the relatively high ρ constant of methanolysis at the benzoyl group substituted derivatives (2.17 and 1.98 for the non‐catalysed and catalysed reactions, respectively), it can be presumed that the transition state structure will be close to the tetrahedral intermediate. Copyright © 1999 John Wiley & Sons, Ltd.