Mechanism of the hydrolysis of 2-phenyl-4H,5H,6H-3-methyl-3-thiazinium perchlorate derivatives

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-methylbenz amide as the only product in the all pH ranges. The observed rate of. hydro lysis of PTP was always of the first-order. For hydrolysis from PTP, Hammet t rho values were 0.53, 0.84 and 1.13 for pH 5.0, 8.0, and 10.0, respective ly. Bronsted beta value was 0.53 for general base catalysis. This reaction is catalyzed by general base at low acetate concentration. However, as the amount of base becomes larger, the rate of hydrolysis reaction approaches t he limiting values. The plot of log k vs. pH shows that the rate constants (k,) are two different regions in the profile; one part is directly proport ional to hydroxide ion concentration and the other is not. On the bases of these results, the plausible hydrolysis mechanism and a reaction equation w ere proposed: Below pH 4.5, the hydrolysis was initiated by the addition of water to alpha-carbon. Above pH 9.0, the hydrolysis was proceeded by the a ddition of hydroxide ion to a-carbon. However, in the range of pH 4.5-8.0, these two reactions occured competitively.