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 (<b>6</b>, <b>7</b>, and <b>8</b>, respectively) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt % ethanol−water, 25.0 °C, ionic strength 0.2 M (KCl). Under excess amine, pseudo-first-order rate coefficients (<i>k</i><sub>obsd</sub>) are obtained for all reactions. Reactions of substrates <b>6 </b>and <b>7</b> with piperidine and of thionocarbonate <b>8</b> with the same amine and piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine show linear <i>k</i><sub>obsd</sub> vs [amine] plots, with slopes (<i>k</i><sub>1</sub>) independent of pH. On the other hand, these plots are nonlinear upward for the reactions of substrates <b>6</b> and <b>7</b> with all the amines, except piperidine, and also for the reactions of compound <b>8</b> with 1-formylpiperazine and piperazinium ion. For all these reactions a mechanistic scheme is proposed with the formation of a zwitterionic tetrahedral intermediate (T<sup>±</sup>), which can transfer a proton to an amine to give an anionic intermediate (T<sup>-</sup>). Rate and equilibrium microcoefficients of this scheme, <i>k</i><sub>1</sub>, <i>k</i><sub>-</sub><sub>1</sub>, <i>K</i><sub>1</sub> (= <i>k</i><sub>1</sub>/<i>k</i><sub>-</sub><sub>1</sub>), and <i>k</i><sub>2</sub>, are obtained by fitting the nonlinear plots through an equation derived from the scheme. The Brönsted-type plots for <i>k</i><sub>1</sub> are linear with slopes β<sub>1</sub> = 0.19, 0.21, and 0.26 for the aminolysis of <b>6</b>,<b> 7</b>, and <b>8</b>, respectively. This is consistent with the hypothesis that the formation of T<sup>±</sup> (<i>k</i><sub>1</sub> step) is the rate-determining step. The <i>k</i><sub>1</sub> values for these reactions follow the sequence <b>8</b> > <b>7</b> > <b>6</b>, consistent with the sequence of the electron-withdrawing effects from the substituents on the “nonleaving” group of the substrates. The <i>k</i><sub>1</sub> values for the aminolysis of <b>6</b>, <b>7</b>, and <b>8</b> are smaller than those for the same aminolysis of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (<b>2</b>, <b>3</b>, and <b>4</b>, respectively). The <i>k</i><sub>2</sub> values (expulsion of the nucleofuge from T<sup>±</sup>) increase as the electron withdrawal from the nonleaving group increases. These values are smaller for the aminolysis of <b>6</b>, <b>7</b>, and <b>8</b> compared to those for the same aminolysis of <b>2</b>, <b>3</b>, and <b>4</b>, respectively.