| Abstract
| - The reactions of the title thionocarbonates (6, 7, and 8, respectively) with a series of secondaryalicyclic amines are subjected to a kinetic investigation in 44 wt % ethanol−water, 25.0 °C, ionicstrength 0.2 M (KCl). Under excess amine, pseudo-first-order rate coefficients (kobsd) are obtainedfor all reactions. Reactions of substrates 6 and 7 with piperidine and of thionocarbonate 8 with thesame amine and piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine show linear kobsd vs[amine] plots, with slopes (k1) independent of pH. On the other hand, these plots are nonlinearupward for the reactions of substrates 6 and 7 with all the amines, except piperidine, and also forthe reactions of compound 8 with 1-formylpiperazine and piperazinium ion. For all these reactionsa mechanistic scheme is proposed with the formation of a zwitterionic tetrahedral intermediate(T±), which can transfer a proton to an amine to give an anionic intermediate (T-). Rate andequilibrium microcoefficients of this scheme, k1, k-1, K1 (= k1/k-1), and k2, are obtained by fittingthe nonlinear plots through an equation derived from the scheme. The Brönsted-type plots for k1are linear with slopes β1 = 0.19, 0.21, and 0.26 for the aminolysis of 6, 7, and 8, respectively. Thisis consistent with the hypothesis that the formation of T± (k1 step) is the rate-determining step.The k1 values for these reactions follow the sequence 8> 7> 6, consistent with the sequence of theelectron-withdrawing effects from the substituents on the “nonleaving” group of the substrates.The k1 values for the aminolysis of 6, 7, and 8 are smaller than those for the same aminolysis of3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (2, 3, and 4,respectively). The k2 values (expulsion of the nucleofuge from T±) increase as the electron withdrawalfrom the nonleaving group increases. These values are smaller for the aminolysis of 6, 7, and 8compared to those for the same aminolysis of 2, 3, and 4, respectively.
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