KINETICS AND MECHANISM OF THE AMINOLYSIS OF PHENYL AND 4-NITROPHENYL CHLOROTHIONOFORMATES

The reactions of a series of secondary alicyclic amines with the title substrates are subjected to a kinetic investigation in aqueous soluti on, 25 degrees C, ionic strength 0.2 M (maintained with KCl). Under am ine excess pseudo-first-order rate coefficients (k(obsd)) are found. T he plots of k(obsd) against concentration of free amine at constant pH are linear, with the slopes (k(N)) independent of pH. The Bronsted-ty pe plots obtained (log k(N) vs amine pK(a)) for the aminolysis of both substrates are linear with the same slope, beta = 0.26. From this val ue, the kinetic law, and the analysis of products, it is deduced that these reactions proceed through a zwitterionic tetrahedral addition in termediate (T+/-) on the reaction path, and its formation is the rate- determining step. From a comparison of the present reactions with the concerted aminolysis of substituted phenyl chloroformates in acetonitr ile it is inferred that the change of S- by O- in T+/- and that of wat er by acetonitrile as solvent destabilizes T+/- in such a way that the stepwise reaction becomes enforced concerted.