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 kinet ic investigation in 44 wt % ethanol-water, 25.0 degreesC, ionic strength 0. 2 M (KCl). Under amine excess over the substrates pseudo-first-order rate c oefficients (k(obsd)) are obtained for all the reactions. Those of the alic yclic 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 in termediates, one zwitterionic (T+/-) and the other anionic (T-), with a kin etically significant proton transfer from T+/- to an amine to give T-. From an equation derived from the scheme the rate microcoefficients are obtaine d through fitting. The rate coefficient for formation of T+/- (k(1)) is lar ger for 1 compared to 2, which can be explained by a stronger electron-with drawal of 4-nitro in 1 than 3-nitro in 2, which leaves the thiocarbonyl car bon of 1 more positive and, therefore, more susceptible to nucleophilic att ack. 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 slop es 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 Ti to products is the rate-determin ing 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.