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 prod ucts are substituted benzamidoximes and methyl benzoates. The slope of the dependence of rate constant on sodium methoxide concentration gradually inc reases, but in the presence of C-18 crown ether the dependence becomes line ar and the rate constant is lower than in the absence of the crown ether, w hich means that the reaction is catalysed by sodium cation. The rate consta nts of reactions with the ion pair and with methoxide ion were determined w ith the presumption that the rate-limited step of the catalysed reaction is the reaction of substituted O-benzoylamidoximes with the ion pair of sodiu m 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 h igh values indicate the rate-limiting step involving the splitting off of s ubstituted benzamidoxime from the tetrahedral intermediate. On the basis of the relatively high rho constant of methanolysis at the benzoyl group subs tituted derivatives (2.17 and 1.98 for the non-catalysed and catalysed reac tions, respectively), it can be presumed that the transition state structur e will be close to the tetrahedral intermediate. Copyright (C) 1999 John Wi ley & Sons, Ltd.