Kinetics and Mechanisms of the Reactions of 4-Nitro- and 3-Nitrophenyl 4-Methylphenyl Thionocarbonates with Alicyclic Amines and Pyridines

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Abstract
The reactions of the title thionocarbonates (1 and 2, respectively) with a series of secondary alicyclic amines and pyridines are subjected to a kinetic investigation in 44 wt % ethanol−water, 25.0 °C, ionic strength 0.2 M (KCl). Under amine excess over the substrates pseudo-first-order rate coefficients (kobsd) are obtained for all the reactions. Those of the alicyclic amines with the two substrates show nonlinear upward plots of kobsd vs [amine], except the reactions of piperidine, which exhibit linear plots. For these reactions a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic (T±) and the other anionic (T-), with a kinetically significant proton transfer from T± to an amine to give T-. From an equation derived from the scheme the rate microcoefficients are obtained through fitting. The rate coefficient for formation of T± (k1) is larger for 1 compared to 2, which can be explained by a stronger electron-withdrawal of 4-nitro in 1 than 3-nitro in 2, which leaves the thiocarbonyl carbon of 1 more positive and, therefore, more susceptible to nucleophilic attack. For the pyridinolyses of both thionocarbonates the plots of kobsd vs [amine] are linear, with the slope (kN) independent of pH. The Brönsted plots (log kN vs pyridine pKa) for these reactions are linear with slopes β = 0.9 and 1.2 for the pyridinolysis of 1 and 2, respectively. These slopes are consistent with a mechanism through a T± intermediate on the reaction path, whereby decomposition of T± to products is the rate-determining step. The kN values are larger for the reactions of 1 than those of 2. This is attributed to a larger equilibrium formation of T± and a larger expulsion rate of the nucleofuge from T± in the reactions of 1 compared to those of 2.

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