Kinetic and mechanistic investigation of the aminolysis of 3-methoxyphenyl 3-nitrophenyl thionocarbonate, 3-chlorophenyl 3-nitrophenyl thionocarbonate, and bis(3-nitrophenyl) thionocarbonate.
The reactions of the title thionocarbonates (6, 7, and 8, respectively) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt % ethanol-water, 25.0 degrees C, ionic strength 0.2 M (KCl). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are obtained for all reactions. Reactions of substrates 6 and 7 with piperidine and of thionocarbonate 8 with the same amine and piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine show linear k(obsd) vs [amine] plots, with slopes (k(1)) independent of pH. On the other hand, these plots are nonlinear upward for the reactions of substrates 6 and 7 with all the amines, except piperidine, and also for the reactions of compound 8 with 1-formylpiperazine and piperazinium ion. For all these reactions a 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 and equilibrium microcoefficients of this scheme, k(1), k(-)(1), K(1) (= k(1)/k(-)(1)), and k(2), are obtained by fitting the nonlinear plots through an equation derived from the scheme. The Brönsted-type plots for k(1) are linear with slopes beta(1) = 0.19, 0.21, and 0.26 for the aminolysis of 6, 7, and 8, respectively. This is consistent with the hypothesis that the formation of T(+/-) (k(1) step) is the rate-determining step. The k(1) values for these reactions follow the sequence 8 > 7 > 6, consistent with the sequence of the electron-withdrawing effects from the substituents on the "nonleaving" group of the substrates. The k(1) values for the aminolysis of 6, 7, and 8 are smaller than those for the same aminolysis of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (2, 3, and 4, respectively). The k(2) values (expulsion of the nucleofuge from T(+/-)) increase as the electron withdrawal from the nonleaving group increases. These values are smaller for the aminolysis of 6, 7, and 8 compared to those for the same aminolysis of 2, 3, and 4, respectively.
Castro
Enrique A
EA
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile. ecastro@puc.cl
Galvez
Angelique
A
Leandro
Leonardo
L
Santos
José G
JG