The reactions of anilines with ethyl S-(2,4-dinitrophenyl) thiocarbonate (DNPTC) and ethyl S-(2,4,6-trinitrophenyl) thiocarbonate (TNPTC) are subjected to a kinetic study in aqueous solution at 25.0 degrees C and ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under aniline excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [N] (N is the free substituted aniline) are linear and pH independent, with slope k(N). The Brönsted-type plots (log k(N) vs pK(a) of anilinium ions) are linear, with slopes beta = 0.9 for DNPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T(+/-)) is rate determining, and beta = 0.54 for TNPTC, consistent with a concerted mechanism. Consideration of the results for aminolysis from the present work and those from previous studies leads to the following conclusions. (i) The tetrahedral intermediate possessing a 2,4-dinitrobenzenethio group is more stable than that including the 2,4,6-trinitrobenzenethio group. (ii) The tetrahedral intermediate possessing the 2,4,6-trinitrobenzenethio group has no existence beyond the limit of a vibration period (concerted mechanism). (iii) Tetrahedral intermediates possessing anilino groups are less stable than those derived from pyridines but are more stable than the tetrahedral intermediates derived from secondary alicyclic amines. (iv) Anilines are more reactive toward the carbonyl group of methyl 2,4-dinitrophenyl carbonate than toward the carbonyl of DNPTC.

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.

The reactions of the title substrate (1) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0ЊC, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (k obs ) are obtained. Plots of k obs against [NH], where NH is the free amine, are nonlinear upwards, except the reactions of piperidine, which show linear plots. According to the kinetic results and the analysis of products, a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic and another anionic with a kinetically significant proton transfer from Ϯ Ϫ (T ) (T ), to an amine to yield (k 3 step). By nonlinear least-squares fitting of an equation derived Ϯ Ϫ T T from the scheme to the experimental points, the rate microcoefficients involved in the reactions are determined. Comparison of the kinetics of the title reactions with the linear k obs vs.[NH] plots found in the same aminolysis of O-ethyl 4-nitrophenyl dithiocarbonate (2) in the same solvent shows that the rate coefficient for leaving group expulsion from (k 2 ) is larger Ϯ T for 2 due to a stronger push by EtO than PhO. The k 3 value is the same for both reactions since both proton transfers are diffusion controlled. Comparison of the title reactions with the same aminolysis of phenyl 4-nitrophenyl thionocarbonate (3) in water indicates that (i) the k 2 value is larger for the aminolysis of 1 due to the less basic nucleofuge involved and the small solvent effect on k 2 , (ii) the k 3 value is smaller for the reactions of 1 due to the more viscous solvent, (iii) the rate coefficient for amine expulsion from is larger for theaminolysis of 1 than that of 3 due to a solvent effect, and (iv) the value of the rate coefficient for amine attack (k 1 ) is smaller for the aminolysis of 1 in aqueous ethanol, which can be explained by a predominant solvent effect relative to the electron-withdrawing effect from the nucleofuge.

The reactions of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (1, 2, and 3, respectively) with a series of secondary alicyclic amines are studied kinetically in 44 wt % ethanol-water at 25.0 degrees C and an ionic strength of 0.2 M (KCl). Pseudo-first-order rate coefficients (k(obsd)) are obtained for all reactions (amine excess was used). The reactions of compound 1 with piperidine, piperazine, and 1-(2-hydroxyethyl)piperazine and of compounds 2 and 3 with these amines and morpholine exhibit linear k(obsd) versus amine concentration plots with slopes (k1) independent of pH. In contrast, the plots are nonlinear upward for the reactions of substrate 1 with morpholine, 1-formylpiperazine, and piperazinium ion and of substrates 2 and 3 with the two latter amines. For all these reactions, a reaction scheme is proposed with a zwitterionic tetrahedral intermediate (T+/-), which can be deprotonated by an amine to yield an anionic intermediate (T-). When the nonlinear plots are fit through an equation derived from the scheme, rate and equilibrium microcoefficients are obtained. The Brönsted-type plots for k1 are linear with slopes of beta1 = 0.22, 0.20, and 0.24 for the aminolysis of 1, 2, and 3, respectively, indicating that the formation of T+/- (k1 step) is rate-determining. The k1 values for these reactions follow the sequence 3 > 2 > 1, which can be explained by the sequence of the electron-withdrawing effects from the substituents on the nonleaving group of the substrates.

No barrier theory has been applied to the E2 reactions of five alkyl bromides with ethanolic ethoxide. The model used for these reactions is that the reaction proceeds from the encounter complex of base and alkyl halide to the product encounter complex of halide ion and alkene (and alcohol), and requires five simple processes, which combine to give the concerted elimination: transfer of a proton from carbon to base; a change in geometry at the carbon which loses a proton from sp 3 to sp 2 ; breaking the C-leaving group bond; a change in geometry at the carbon which loses the leaving group from sp 3 to sp 2 ; and a change in the length of the carbon-carbon bond. The free energy of activation can be calculated with an rms error of 2.58 kcal mol -1 (1 cal = 4.184 J).Résumé : On a appliqué la théorie sans barrière à l'étude des réactions E2 de cinq bromures d'alkyles avec de l'éthanolate éthanolique. Le modèle utilisé pour ces réactions est celui d'une réaction qui se produit à partir d'un complexe de rencontre de la base et de l'halogénure d'alkyle et qui conduit au complexe de rencontre du produit avec l'ion halogénure, l'alcène (et l'alcool) et qui nécessite cinq processus simples qui se combinent pour conduit à une éli-mination concertée: le transfert d'un proton du carbone à la base; une modification de la géométrie au niveau du carbone qui perd un hydrogène et qui passe de sp 3 à sp 2 ; un changement de géométrie au niveau du carbone qui perd le groupe partant et qui passe de sp 3 à sp 2 ; la rupture de la liaison du carbone-groupe partant et un changement dans la longueur de la liaison carbone-carbone. On a calculé l'énergie libre d'activation avec une erreur de 2,58 kcal/mol (1 cal = 4,184 J).