KINETICS AND MECHANISM OF THE AMINOLYSIS OF O-ETHYL S-ARYL DITHIOCARBONATES

The reactions of O-ethyl S-phenyl dithiocarbonate (1) and O-ethyl S-(p-nitrophenyl) dithiocarbonate (2) with a series of secondary alicyclic amines, namely, piperidine, piperazine, 1-(beta-hydroxyethyl)piperazine, morpholine, 1-formylpiperazine, and (with 2 only) piperazinium ion, are subject to a kinetic study at several pH values. The reaction leads to the corresponding thiocarbamates and thiophenols (measured as thiophenoxide ion by UV-vis spectrophotometry). Pseudo-first-order rate coefficients (k(obsd)) are found throughout (amine excess). The kinetics are first order in amine for the reactions of 2. The plots of k(obsd) vs [amine] for the reactions of 1, except with 1-formylpiperazine, are linear, but near the origin they are curved, showing a more complex rate equation. The reaction of 1 with 1-formylpiperazine shows a second-order dependence on the amine. No dependence on pH of the second-order rate constant values is observed. The findings are well-accommodated by a mechanistic model involving reversible nucleophilic attack on the thiocarbonyl group, two tetrahedral intermediates, 3 and 4, and a deprotonation step. The Bronsted-type plots obtained are linear (beta-1 = 0.22) for the reaction of 1 and curved for 2 (beta-1 = 0.2 and beta-2 = 0.8). The Bronsted-type plot obtained with the rate constants for amine expulsion from 3 is linear with beta-1 = -0.67 and -0.54 for the reactions of 1 and 2, respectively.