Kinetics and mechanisms of the reactions of 4-nitro- and 3-nitrophenyl 4-methylphenyl thionocarbonates with alicyclic amines and pyridines
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 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrates pseudo-first-order rate coefficients (k(obsd)) are obtained for all the reactions. Those of the alicyclic amines with the two substrates show nonlinear upward plots of k(obsd) 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(+/-) (k(1)) 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 k(obsd) vs [amine] are linear, with the slope (k(N)) independent of pH. The Bronsted plots (log k(N) vs pyridine pK(a)) for these reactions are linear with slopes beta = 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 k(N) 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.