Stepwise Mechanism of the Aminolysis of O-Ethyl S-(4-Nitrophenyl) Thiocarbonate

Abstract

The reactions of secondary alicyclic amines with the title substrate (NPTC) are subjected to a kinetic study in water, 25.0 o C, ionic strength 0.2 M (KCl). The reactions are first order in both the amine and NPTC. The Bronsted-type plot for the second-order rate coefficient is biphasic, with slopes β 1 =0.2 and β 2 =0.8 at high and low pK a values, respectively. The Bronsted plot is explained by the presence of a zwitterionic tetrahedral intermediate and a change in the rate-determining step. The center of the Bronsted curvature is pK a o =10.7. A semiempirical equation based on the above hypothesis accounts for the experimental results. The value of pKa°=10.7 is larger than those found in the same aminolyses of O-ethyl 4-nitrophenyl dithiocarbonate and S-(4-nitrophenyl)thioacetate, favoring amine expulsion from the tetrahedral intermediate (relative to 4-nitrobenzenethiolate ion expulsion) in the NPTC reaction. By comparison of the stepwise reaction under study with the concerted aminolysis of O-ethyl S-(2,4-dinitrophenyl) thiocarbonate it is concluded that the introduction of a second nitro group to the tetrahedral intermediate formed in the present reactions destabilizes the intermediate to such an extent that it becomes a transition state and a concerted mechanism is enforced