Kinetics and Mechanism of the Aminolysis of O-Ethyl S-Phenyl Dithiocarbonate in Aqueous Ethanol

Article abstract of DOI:10.1002/kin.550270107

The reactions of secondary alicyclic amines with the title substrate (PDTC) are subjected to a kinetic study in 44 wt.percent aqueous ethanol, 25.0 deg C, ionic strength 0.2 M (KCl).Pseudo-first-order rate coefficients (k_obs) are found under amine excess.Linear plots of /k_obs against 1/, where N is the free amine, are obtained for the reactions with piperidine, piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine.The reaction with 1-formylpiperazine exhibits a linear plot of k_obs against ².These results are interpreted through a mechanism consisting of two tetrahedral intermediates: a zwitterionic (T^+/-) and an anionic (T^-), where the amine catalyzed proton transfer from T^+/- to T^- is partially rate determining for the four former reactions and is fully rate determining for the reaction of 1-formylpiperazine.The rate microcoefficients involved in the reaction scheme are either determined experimentally or estimated.Comparison with the corresponding microcoefficients reported for the same reactions in water reveals that the rate coefficient for formation of T^+/- from reactants (k₁) is smaller and that for the reversal of this (k_-1) is larger in aqueous ethanol compared to water, in agreement with the expected structure of the corresponding transition state.Broensted-type plots are obtained for k₁, k_-1, and K₁ (= k₁/k_-1) with slopes ca. 0.4, -0.6, and 1.0, respectively.Comparison of the present stepwise reactions with the concerted ones found in the same aminolysis of O-ethyl 2,4,6-(trinitrophenyl) dithiocarbonate indicates that T^+/- is so destabilized by the change of PhS by the 2,4,6-trinitrobenzenethio group that T^+/- no longer exists and becomes a transition state.