Kinetics of methanolysis of substituted benzamide O-(phenoxycarbonyl)- andO-(alkoxycarbonyl)oximes catalyzed by sodium methanolate

The kinetics of methanolysis of substituted benzamide O-(phenoxycarbonyl) - and O-(alkoxycarbonyl)oximes catalyzed by sodium methanolate was studied a t 25 degrees C. The reaction proceeds in two steps. In the first, faster st ep, the substituted phenoxy group is exchanged for a methoxy group giving r ise to substituted O-(methoxycarbonyl)oximes. In the second step, a benzami de oxime is eliminated and dimethyl carbonate is formed. The slope of the p lot of the rate constant in dependence on the sodium methanolate concentrat ion has an increasing tendency in both steps. In the presence of 18-crown-6 , the plots are linear and the rate constants are lower than in the absence of the crown ether. The rate constants of the reaction of the substrate wi th the methanolate ion and with the MeONa ion pair were determined assuming that the sodium cation-catalyzed reactions constitute the rate-determining step of the reaction of the substrate with the MeONa ion pair. For the eli mination of the aryloxy group and of the substituted benzamide oxime, the r ate constants of the reaction with the ion pair are roughly twelvefold and twentyfold higher, respectively, than in the uncatalyzed reaction. The slop e of the dependence of log k on the pK(a) of the substituted phenols (beta( lg)) has the value of -0.52 for the uncatalyzed reaction of elimination of the substituted phenoxy group, -0.83 for the elimination of the benzamide o xime group, and -0.53 for the reaction with the ion pair. In the first step and probably also in the second step, the reaction proceeds by the concert ed mechanism. The relatively high rho value of methanolysis of substituted benzamide O-(4-nitrophenoxycarbonyl)oximes, 0.63, suggests that the structu re of the transition state approaches that of the tetrahedral intermediate.