Reactions Of Secondary Alicyclic Amines Research Articles

Kinetics and mechanism of the aminolysis of bis(4‐nitrophenyl) carbonate and O‐(4‐nitropheny) S‐(4‐nitrophenyl) thio and dithiocarbonate

The kinetics of the reactions of secondary alicyclic amines with bis(4‐nitrophenyl) carbonate (1), S‐(4‐nitrophenyl) O‐(4‐nitrophenyl) thiocarbonate (2) and S‐(4‐nitrophenyl) O‐(4‐nitrophenyl) dithiocarbonate (3) were studied at 25.0 °C. The reactions of 1 were studied in water and those of 2 and 3 in 44 wt% ethanol–water. These reactions are concerted. The HPLC analyses of the reactions of 2 and 3 with morpholine show that both 4‐nitrophenoxide and 4‐nitrobenzenethiolate ions are the nucleofuges. Copyright © 2013 John Wiley & Sons, Ltd.

Reactions of aryl acetates with secondary alicyclic amines in ethanol/water mixtures: Effect of the solvent composition on the kinetics and mechanism

AbstractWe report a kinetic study on the reactions of secondary alicyclic amines toward 4‐nitrophenyl, 2,4‐dinitrophenyl, and 2,4,6‐trinitrophenyl acetates (1, 2, and 3) in ethanol/water mixtures of different compositions. It is found that (i) the intermediate in the reaction of 1 is stabilized in a mixture of 90 vol% ethanol; (ii) for the reaction of 2, the mechanism is stepwise in water but concerted in the mixtures; (iii) For the reaction of 3, the mechanism is concerted along the whole range of composition; (iv) the effect of NO2 outweighs the solvent effect; (v) preferential solvation in the core of reaction can be ruled out. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 687–693, 2011

Kinetics and mechanism of the reactions of S‐4‐nitrophenyl 4‐methylthiobenzoate with secondary alicyclic amines and pyridines

AbstractThe reactions of the title substrate with a series of six secondary alicyclic amines and a series of eight pyridines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0°C, and an ionic strength of 0.2 mol · dm−3. Under amine excess pseudo first‐order rate coefficients (kobs) are obtained. Plots of kobs against free amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Brønsted‐type plots (log kN against pKa of the conjugate acids of the amines) are non‐linear, with the curvature center located at pKa (pK) 9.7 and 9.4, for the reactions of secondary alicyclic amines and pyridines, respectively. The plots are consistent with a zwitterionic tetrahedral intermediate (T±) on the reaction path and a change in rate‐determining step. The greater pK value for secondary alicyclic amines than pyridines is explained by the greater nucleofugality from T± of secondary alicyclic amines compared to isobasic pyridines. These pK values are lower than those found for the reactions of S‐4‐nitrophenyl 4‐Y‐substituted thiolbenzoates (Y = H, Cl, NO2) with the corresponding amine series. These results indicate that electron donation from the non‐leaving group in T± favors leaving group expulsion from T± relative to amine expulsion. Copyright © 2006 John Wiley & Sons, Ltd.

Kinetics and mechanism of the pyridinolysis of s-4-nitrophenyl 4-substituted thiobenzoates in aqueous ethanol.

The pyridinolysis of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO2; 1, 2, and 3, respectively) is studied kinetically in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions are measured spectrophotometrically (420-425 nm) by following the appearance of 4-nitrobenzenethiolate anion. Pseudo-first-order rate coefficients (kobsd) are obtained throughout, under excess of amine over the substrate. Plots of kobsd vs [free amine] at constant pH are linear with the slope (kN) independent of pH. The Brnsted-type plot (log kN vs pKa0 of the conjugate acids of the pyridines) for the reactions of thiolbenzoate 1 is curved with a slope at high pKa, beta1 = 0.20, and slope at low pKa0, beta2 = 0.94. The pKa value for the center of the Brnsted curvature is pKa0 = 9.7. The pyridinolysis of thiolbenzoates 2 and 3 show linear Brnsted-type plots of slopes 0.94 and 1.0, respectively. These results and other evidence indicate that these reactions occur with the formation of a zwitterionic tetrahedral intermediate (T+/-). For the pyridinolysis of thiolbenzoate 1, breakdown of T+/- to products (k2 step) is rate-limiting for weakly basic pyridines and T+/- formation (k1 step) is rate-determining for very basic pyridines. The k2 step is rate-limiting for the reactions of thiolbenzoates 2 and 3. The smallest pKa0 value for the reaction of 1 is due to the weakest electron withdrawal of H (relative to Cl and NO2) in the acyl group, which results in the smallest k-1/k2 ratio. The pKa0 values for the title reactions are smaller than those for the reactions of secondary alicyclic amines with thiolbenzoates 1-3. This is attributed to a lower leaving ability from the T+/- of pyridines than isobasic alicyclic amines. The lower p value found for the pyridinolysis of 2,4-dinitrophenyl benzoate (pKa0 = 9.5), compared with that for the pyridinolysis of 1, is explained by the greater nucleofugality from T+/- of 2,4-dinitrophenoxide than 4-nitrobenzenethiolate, which renders the k-1/k2 ratio smaller for the reactions of the benzoate relative to thiolbenzoate 1. The title reactions are also compared with the aminolysis of similar thiolbenzoates in other solvents to assess the solvent effect.

Kinetics and mechanism of the phenolysis of asymmetric diaryl carbonates.

The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0 degrees C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Brönsted-type plots for the nucleophilic rate constants (k(N)) are linear, with slopes beta = 0.48 (MPNPC), 0.67 (ClPNPC), 0.41 (MPDNPC), and 0.32 (ClPDNPC). The magnitude of these slopes and the absence of a curvature in the Brönsted plot at pK(a) = 7.1 for the CIPNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and ClPDNPC are more reactive than MPNPC and CIPNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log k(N) in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log k(N) to the basicity of the nonleaving moiety (beta(nlg) = -0.27) is very similar to that of the nucleofuge (beta(lg) = -0.25).

Concerted mechanisms of the reactions of 2,4,6-trinitrophenyl methyl carbonate and 2,4,6-trinitrophenyl acetate with secondary alicyclic amines.

The reactions of secondary alicyclic amines with 2,4,6-trinitrophenyl methyl carbonate (TNPMC) and 2,4,6-trinitrophenyl acetate (TNPA) are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of the 2,4,6-trinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [amine] are linear, with the slope (kN) independent of pH. The Brönsted-type plots (log k(N) vs pK(a) of the conjugate acid of the amines) are linear, with slopes beta = 0.41 and beta = 0.36 for the reactions of TNPA and TNPMC, respectively. The predicted breaks of the Brönsted plots for stepwise mechanisms are pK(a)0 = 6.8 and 7.3, respectively. The lack of Brönsted breaks for these reactions and the values of the Brönsted slopes are consistent with concerted mechanisms. By comparison of the reactions under investigation among them and with similar aminolysis and pyridinolysis, the following conclusions can be drawn: (i) Secondary alicyclic amines react with TNPA and TNPMC by concerted mechanisms. (ii) TNPA is more reactive toward these amines than TNPMC due to the greater electron release of MeO from the latter substrate. (iii) The change of 2,4-dinitrophenoxy to 2,4,6-trinitrophenoxy in the zwitterionic tetrahedral intermediate (T+/-) formed in the reactions of the title amines with 2,4-dinitrophenyl acetate greatly destabilizes T+/-. (iv) Secondary alicyclic amines destabilize T+/- relative to pyridines. (v) The intermediate T+/- formed in the reactions of the title amines with S-(2,4,6-trinitrophenyl) acetate is greatly destabilized by substitution of S-(2,4,6-trinitrophenyl) by O-(2,4,6-trinitrophenyl) as the leaving group.

Structure−Reactivity Correlations in the Aminolysis of Ethyl S-Aryl Thiolcarbonates

The reactions of secondary alicyclic amines (saa) with ethyl S-4-X-phenyl thiolcarbonates (X = Cl, H, Me, and MeO) are subjected to a kinetic analysis in water, 25.0 °C, ionic strength 0.2 (KCl). B...

Kinetics and Mechanism of the Aminolysis of Phenyl and Methyl 4-Nitrophenyl Thionocarbonates.

The reactions of secondary alicyclic amines with the title substrates are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl), by following spectrophotometrically the release of 4-nitrophenoxide ion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. For the reactions of phenyl 4-nitrophenyl thionocarbonate (1), linear plots of k(obsd) vs [NH] (NH is the free amine) are obtained, except for the reaction with piperazinium ion, which shows nonlinear upward plots. The aminolysis of methyl 4-nitrophenyl thionocarbonate (2) exhibits nonlinear plots of k(obsd) vs [NH], except that with piperidine, which is linear. The Brönsted-type plot for 1 is linear with slope beta = 0.25, indicating that the formation (k(1) step) of a tetrahedral addition intermediate (T(+/-)) is rate determining. For the aminolysis of 2 (except piperidine), k(-)(1) approximately k(3)[NH] > k(2), where k(-)(1), k(3), and k(2) are the rate coefficients for amine expulsion, amine deprotonation, and leaving group expulsion from T(+/-), respectively. For the reaction of 2 with piperidine, k(-)(1) < k(3)[NH]; therefore, the k(1) step is rate limiting. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) The change of MeO by EtO in 2 does not affect the k(1), k(-)(1), or k(2) values. (ii) Substitution of MeO by PhO in 2 results in lower k(1) values due to steric hindrance. (iii) The change of 4-nitrophenoxy (NPO) by PhO in 2 lowers the k(1) values and enlarges those of k(-)(1). (iv) Secondary alicyclic amines are less reactive toward 2 than isobasic pyridines when the breakdown of T(+/-) is rate determining; this is mainly due to larger k(-)(1) values for the former amines. (v) The change of PhO by NPO in 1 changes the mechanism from stepwise to concerted. (vi) Substitution of NPO by PhO in 1 does not alter the k(1) values significantly. (vii) The change of NPO by Cl in 1 increases the k(1) values. (viii) Substitution of C=S by C=O in 1 shifts the rate-limiting step from k(1) to k(2) due to a larger k(-)(1)/k(2) ratio by this change.

Kinetics and Mechanism of the Aminolysis of Phenyl and 4-Nitrophenyl Chloroformates in Aqueous Solution.

The reactions of secondary alicyclic amines with phenyl and 4-nitrophenyl chloroformates (PClF and NPClF, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions are followed spectrophotometrically at 210-270 nm (PClF) and at 310-400 nm (NPClF). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. From linear plots of k(obsd) vs free amine concentration, the second-order rate coefficients (k(N)) for aminolysis are obtained. For the aminolysis of both substrates, linear Brönsted-type plots (log k(N) vs amine pK(a)) of slopes 0.23 (PClF) and 0.26 (NPClF) are found. The values of the slopes are consistent with stepwise mechanisms where the formation of a zwitterionic tetrahedral intermediate (T(+/-)) is the rate-determining step (k(1) step). In contrast, the aminolysis (anilines) of the same substrates in acetonitrile are concerted, which is attributed to destabilization of T(+/-) in the latter solvent due to a faster expulsion of the amine from T(+/-) in acetonitrile compared to water. The values of k(1) are larger for the title reactions compared to the same aminolysis of the corresponding thionochloroformates, and this is attributed to the relatively hard character of these amines which prefer to bind to the harder carbonyl group (relative to thiocarbonyl). There is no change in mechanism by the change of S(-) by O(-) in T(+/-), which should destabilize this intermediate. By comparison with the stepwise pyridinolysis of methyl chloroformate, it is concluded that the changes of methoxy by phenoxy (or 4-nitrophenoxy) and a pyridine by an alicyclic amine in T(+/-) do not greatly affect the stability of these intermediates.

Kinetics and mechanism of the aminolysis of O‐phenyl 4‐nitrophenyl dithiocarbonate in aqueous ethanol

The reactions of secondary alicyclic amines with the title substrate (PDTC) are subjected to a kinetic study in 44 wt.% aqueous ethanol, 25.0°C, ionic strength 0.2 M (KCl). Pseudo-first-order rate coefficients (kobs) are found under amine excess. Linear plots of [N]/kobs against 1/[N], where N is the free amine, are obtained for the reactions with piperidine, piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine. The reaction with 1-formylpiperazine exhibits a linear plot of kobs against [N]2. These results are interpreted through a mechanism consisting of two tetrahedral intermediates: a zwitterionic (T±) and an anionic (T−), where the amine catalyzed proton transfer from T± to T− is partially rate determining for the four former reactions and is fully rate determining for the reaction of 1-formylpiperazine. The rate microcoefficients involved in the reaction scheme are either determined experimentally or estimated. Comparison with the corresponding microcoefficients reported for the same reactions in water reveals that the rate coefficient for formation of T± from reactants (k1) is smaller and that for the reversal of this (k−1) is larger in aqueous ethanol compared to water, in agreement with the expected structure of the corresponding transition state. Bronsted-type plots are obtained for k1, k−1, and K1 (=k1/k−1) with slopes ca. 0.4, −0.6, and 1.0, respectively. Comparison of the present stepwise reactions with the concerted ones found in the same aminolysis of O-ethyl 2,4,6,-(trinitrophenyl) dithiocarbonate indicates that T± is so destabilized by the change of PhS by the 2,4,6-trinitrobenzenethio group that T± no longer exists and becomes a transition state. © 1995 John Wiley & Sons, Inc.

Structure−Reactivity Correlations in the Aminolysis and Pyridinolysis of Bis(phenyl) and Bis(4-nitrophenyl) Thionocarbonates

The reactions of a series of secondary alicyclic amines with bis(phenyl) and bis(4-nitrophenyl) thionocarbonates (BPTOC and BNPTOC, respectively), and a series of pyridines with the latter substrate, are subjected to a kinetic investigation in water, 25.0 °C, ionic strength 0.2 M (KCl). All the reactions obey pseudo-first-order kinetics under amine excess over the substrate. The reactions of piperidine with BPTOC are first order in amine, but those of the same substrate with the other secondary alicyclic amines exhibit a complex order, consistent with the existence of both a zwitterionic (T±) and an anionic (T-) tetrahedral intermediates on the reaction pathway. Deprotonation of T± by a secondary amine to give T- (rate coefficient k3) competes with expulsion of the amine moiety from T± (k-1), except in the reactions of 1-formylpiperazine whereby k-1 ≫ k3 [1-formylpiperazine], and a kinetics second order in amine is observed. The reactions of secondary alicyclic amines with BNPTOC are all first order in am...

Kinetics and mechanism of the aminolysis of O‐ethyl S‐phenyl dithiocarbonate in aqueous ethanol

AbstractThe reactions of secondary alicyclic amines with the title substrate (PDTC) are subjected to a kinetic study in 44 wt.% aqueous ethanol, 25.0°C, ionic strength 0.2 M (KCl). Pseudo‐first‐order rate coefficients (kobs) are found under amine excess. Linear plots of [N]/kobs against 1/[N], where N is the free amine, are obtained for the reactions with piperidine, piperazine, 1‐(2‐hydroxyethyl)piperazine, and morpholine. The reaction with 1‐formylpiperazine exhibits a linear plot of kobs against [N]2. These results are interpreted through a mechanism consisting of two tetrahedral intermediates: a zwitterionic (T±) and an anionic (T−), where the amine catalyzed proton transfer from T± to T− is partially rate determining for the four former reactions and is fully rate determining for the reaction of 1‐formylpiperazine. The rate microcoefficients involved in the reaction scheme are either determined experimentally or estimated. Comparison with the corresponding microcoefficients reported for the same reactions in water reveals that the rate coefficient for formation of T± from reactants (k1) is smaller and that for the reversal of this (k−1) is larger in aqueous ethanol compared to water, in agreement with the expected structure of the corresponding transition state.Bronsted‐type plots are obtained for k1, k−1, and K1 (=k1/k−1) with slopes ca. 0.4, −0.6, and 1.0, respectively. Comparison of the present stepwise reactions with the concerted ones found in the same aminolysis of O‐ethyl 2,4,6,‐(trinitrophenyl) dithiocarbonate indicates that T± is so destabilized by the change of PhS by the 2,4,6‐trinitrobenzenethio group that T± no longer exists and becomes a transition state. © 1995 John Wiley &amp; Sons, Inc.

Stepwise Mechanism of the Aminolysis of O-Ethyl S-(4-Nitrophenyl) Thiocarbonate

The reactions of secondary alicyclic amines with the title substrate (NPTC) are subjected to a kinetic study in water, 25.0 o C, ionic strength 0.2 M (KCl). The reactions are first order in both the amine and NPTC. The Bronsted-type plot for the second-order rate coefficient is biphasic, with slopes β 1 =0.2 and β 2 =0.8 at high and low pK a values, respectively. The Bronsted plot is explained by the presence of a zwitterionic tetrahedral intermediate and a change in the rate-determining step. The center of the Bronsted curvature is pK a o =10.7. A semiempirical equation based on the above hypothesis accounts for the experimental results. The value of pKa°=10.7 is larger than those found in the same aminolyses of O-ethyl 4-nitrophenyl dithiocarbonate and S-(4-nitrophenyl)thioacetate, favoring amine expulsion from the tetrahedral intermediate (relative to 4-nitrobenzenethiolate ion expulsion) in the NPTC reaction. By comparison of the stepwise reaction under study with the concerted aminolysis of O-ethyl S-(2,4-dinitrophenyl) thiocarbonate it is concluded that the introduction of a second nitro group to the tetrahedral intermediate formed in the present reactions destabilizes the intermediate to such an extent that it becomes a transition state and a concerted mechanism is enforced

Concerted mechanism of the reactions of secondary alicyclic amines with O-ethyl S-(2,4,6-trinitrophenyl) thiocarbonate

The title reactions are subjected to a kinetic study in water, 25.0 o C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order coefficients (k obsd ) are obtained. Plots of k obsd vs free amine concentration at constant pH are linear, with the slope (k N ) independent of pH. The Bronsted-type plot (log k N vs pK a of the amine) found is linear, with slope β=0.48, indicating a concerted mechanism