Mechanism of the Hydrolysis of 2-Phenyl-4H,5H,6H-3-methyl-3-thiazinium Perchlorate Derivatives

摘要

Hydrolysis reactions of 2-phenyl-4H,5H,6H-3-methyl-3-thiazinium perchlorate (PTP) and its derivatives at various pH have been investigated kinetically. The hydrolysis is quantitative, producing N-3-mercaptopropyl-N-methylbenzamide as the only product in the all pH ranges. The observed rate of hydrolysis of PTP was always of the first-order. For hydrolysis from PTP, Hammett ヱvalues were 0.53, 0.84 and 1.13 for pH 5.0, 8.0, and 10.0, respectively. Bronsted モvalue was 0.53 for general base catalysis. This reaction is catalyzed by general w acetate concentration. However, as the amount of base becomes larger, the rate of hydrolysis reaction approaches the limiting values. The plot of log k vs. pH shows that the rate constants (kt) are two different regions in the profile; one part is directly proportional to hydroxide ion concentration and the other is not. On the bases of these result, the plausible hydrolysis mechanism and a reaction equation were proposed: Below pH 4.5, the hydrolysis was initiated by the addition of water to メ-carbon. Above pH 9.0, the hydrolysis was proceeded by the addition of hydroxide ion to メ-carbon. However, in the range of pH 4.5-8.0, these two reactions occured competitively.