Abstract

The reactions of the title substrate (1) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0 °C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (kobs) are obtained. Plots of kobs against [NH], where NH is the free amine, are nonlinear upwards, except the reactions of piperidine, which show linear plots. According to the kinetic results and the analysis of products, a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic (T±) and another anionic (T-), with a kinetically significant proton transfer from T± to an amine to yield T- (k3 step). By nonlinear least-squares fitting of an equation derived from the scheme to the experimental points, the rate microcoefficients involved in the reactions are determined. Comparison of the kinetics of the title reactions with the linear kobs vs. [NH] plots found in the same aminolysis of O-ethyl 4-nitrophenyl dithiocarbonate (2) in the same solvent shows that the rate coefficient for leaving group expulsion from T± (k2) is larger for 2 due to a stronger push by EtO than PhO. The k3 value is the same for both reactions since both proton transfers are diffusion controlled. Comparison of the title reactions with the same aminolysis of phenyl 4-nitrophenyl thionocarbonate (3) in water indicates that (i) the k2 value is larger for the aminolysis of 1 due to the less basic nucleofuge involved and the small solvent effect on k2, (ii) the k3 value is smaller for the reactions of 1 due to the more viscous solvent, (iii) the rate coefficient for amine expulsion from T± (k-1) is larger for the aminolysis of 1 than that of 3 due to a solvent effect, and (iv) the value of the rate coefficient for amine attack (k1) is smaller for the aminolysis of 1 in aqueous ethanol, which can be explained by a predominant solvent effect relative to the electron-withdrawing effect from the nucleofuge.