Kinetics and mechanism of the reaction of substituted O-benzoylbenzamidoximes with sodium methoxide in methanol

The kinetics of reaction of substituted O-benzoylbenzamidoximes with sodium methoxide in methanol were studied at 25 degrees 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 pK(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 rho 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 (C) 1999 John Wiley & Sons, Ltd.