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 solution, 25 degrees C, ionic strength 0.2 M (maintained with KCl). Under amine excess pseudo-first-order rate coefficients (k(obsd)) are found. The plots of k(obsd) against concentration of free amine at constant pH are linear, with the slopes (k(N)) independent of pH. The Brönsted-type 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 value, the kinetic law, and the analysis of products, it is deduced that these reactions proceed through a zwitterionic tetrahedral addition intermediate (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 acetonitrile it is inferred that the change of S(-) by O(-) in T(+/-) and that of water by acetonitrile as solvent destabilizes T(+/-) in such a way that the stepwise reaction becomes enforced concerted.