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Relevant academic research and scientific papers

Kinetics and Mechanism of the Aminolysis of O-Ethyl S-Phenyl Dithiocarbonate in Aqueous Ethanol

The reactions of secondary alicyclic amines with the title substrate (PDTC) are subjected to a kinetic study in 44 wt.percent aqueous ethanol, 25.0 deg C, ionic strength 0.2 M (KCl).Pseudo-first-order rate coefficients (kobs) are found under amine excess.Linear plots of /kobs against 1/, 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 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.Broensted-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.

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). By following the leaving groups spectrophotometrically (260-270 nm), under amine excess, pseudo-first-order rate coefficients (kobsd) are obtained. Plots of kobsd against free-amine concentration at constant pH are linear, with slope (kN) independent of pH. The Broensted-type plots (log kN against amine pKa) are linear for the aminolysis of the four substrates, with slopes βN = 0.7-0.8. The magnitudes of the slopes are consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T±) whose breakdown to products is rate-determining (k2 step). This mechanism is simpler than that for the same aminolysis of the corresponding ethyl S-aryl dithiocarbonates, which includes proton transfer from T± to the amine (k3 step), due to the fact that for the title reactions k2 > k3[amine]. With the results of the present work and those for the reactions of saa with ethyl S-4-nitrophenyl thiolcarbonate, a dual parametric equation is deduced for kN as a function of amine and leaving group basicity, with both βN and βlg = 0.8. Another dual parametric equation is deduced from literature data for the pyridinolysis of ethyl S-aryl thiolcarbonates. Comparison of both multiparametric expressions shows that pyridines are more reactive than isobasic saa toward thiolcarbonates when breakdown of T± to products is rate-limiting.

Kinetics and mechanism of the aminolysis of phenyl and 4-nitrophenyl ethyl thionocarbonates

The reactions of the title substrates (PTOC and NPTOC, respectively) with secondary alicyclic amines are subjected to a kinetic study in aqueous solution at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients (kobsd) are found throughout. The order in amine is one for the reactions of piperidine but is of intermediate order between 1 and 2 for the reactions of the other amines. The kinetic results can be accommodated by a reaction scheme with two hypothetical tetrahedral intermediates: a zwitterionic (T±) and an anionic (T-) one, whereby amine catalysis (deprotonation of T± to give T-) is kinetically important. Both the pKa of T± and the rate coefficient for proton transfer (k3 ca. 1010 s-1 M-1) are estimated. The values of the other rate microcoefficients of the scheme are found by a nonlinear least-squares fitting, and these values are compared with those exhibited in the aminolysis of phenyl thionoacetate (PTOA), and S-phenyl and S-(4-nitrophenyl) O-ethyl dithiocarbonates (PDTC and NPDTC, respectively). The Broensted type plots for amine basicity have slopes βN ca. 0.2 for rate-determining amine attack (k1) and βN ca. 0.8 for amine expulsion from T± (k-1), in accord with the βN values found in similar aminolyses. The general base catalysis by amine found in the aminolysis of NPTOC, in contrast with the lack of such catalysis in the aminolysis of 4-nitrophenyl methyl carbonate, is explained by a smaller rate coefficient for expulsion of 4-nitrophenoxide (k2) from T±(which competes with amine deprotonation of T±) relative to the same expulsion from the analogous oxy intermediate.

Stepwise Mechanisms of the Aminolyses of 4-Nitrophenyl and 2,4-Dinitrophenyl O-Ethyl Dithiocarbonates in Aqueous Ethanol

The reactions of the title substrates with a series of secondary alicyclic amines are subjected to a kinetic study in 44 wtpercent aqueous ethanol, at 25.0 deg C, ionic strength 0.2 M (maintained with KCl).Pseudo-first-order kinetics are found under amine excess.The order in amine obtained is one in the reactions of all amines.The Broensted-type plots for the overall second-order rate coefficients are biphasic with slopes β1 = 0.3 (high pKa), β2 = 0.95 (low pKa) for the aminolysis of the 4-nitro derivative, and β1 = 0.3 and β2 = 0.80 for the aminolysis of the dinitro compound; the pKa values at the curvature center are pK0a 9.8 and 9.5, respectively.From a comparison of these pK0a values with those found in the same reactions in water and the shapes of the Broensted-type plots, it is concluded that these reactions are stepwise, with the formation of a zwitterionic tetrahedral intermediate in the reaction pathway.

Structure-Reactivity Relationships in the Aminolysis of O-Ethyl S-Aryl Dithiocarbonates in Aqueous Solution

The reactions of O-ethyl S-(X-phenyl) diothiocarbonates (X = p-Cl, p-Me, and p-MeO) with a series of secondary alicyclic amines and those of the same substrates and analogous derivatives (X = H, p-NO2, 2,4-(NO2)2 with pyrrolidine are subjected to a kinetic study in water, 25.0 deg C, ionic strength 0.2 M (KCl).The reactions of piperidine and pyrrolidine show second-order kinetics (first order in amine) with the formation of a zwitterionic tetrahedral intermediate (T+/-) as the rate-determining step.The reactions of the other amines exhibit orders in amine differentfrom one, compatible with the presence of an anionic intermediate (T-), resulting from a kinetically important proton transfer from T+/- to the amine.The rate of this proton transfer is faster than that of expulsion of arylthiolate from T+/- (k2) as evidenced by the estimation of these rate coefficients.The rate constants for formation of T+/- (k1) are obtained experimentally, and those for amine expulsion from T+/- (k-1) are estimated.Equations for k1 and k-1 are derived as functions of the basicity of the amine and leaving (aryl thiolate) groups.Comparison of the k-1 and k2 values found in the present reactions with those obtained in the aminolysis of aryl dithioacetates shows that substitution of Me by EtO as the "acyl" group of the zwitterionic tetrahedral intermediate destabilizes this species.

Kinetics and Mechanism of the Aminolysis of 2,4-Dinitrophenyl and 2,4,6-Trinitrophenyl O-Ethyl Dithiocarbonates

The reactions of a series of secondary alicyclic amines with the title substrates (DNPDTC and TNPDTC) are subject to a kinetic study in water at 25.0 deg C, ionic strength 0.2 M (KCl).The reactions are first order in both the amine and the substrate.The Broensted-type plots obtained are nonlinear and are explained by a stepwise mechanism through a zwitterionic tetrahedral intermediate (T+/-), although a concerted process is not rigorously excluded for the reactions of the less basic amines with TNPDTC.An equation is reported which describes the dependence of the basicities of the amine and leaving group on the nucleofugality ratio of these groups from T+/- (k-1/k2).This equation satisfactorily predicts the pKa values at the Broensted breaks (pKa0).The experimental pKa0. values found in the present reactions correlate well with those in the aminolysis of p-nitro- and unsubstituted phenyl O-ethyl dithiocarbonates.The values of both k-1 and k2 are larger in the reactions of phenyl O-ethyl dithiocarbonate than in the aminolysis of phenyl dithioacetate due to the additional push exerted by EtO in T+/-.The T+/- formed in the aminolysis of DNPDTC is less unstable than that in the aminolysis of S-(2,4-dinitrophenyl)O-ethyl thiocarbonate (whose mechanism is concerted), and this is attributed to a smaller ability of S- in T+/- to form a double bond and expel a nucleofuge.

Kinetics and Mechanism of the Aminolysis of O-Ethyl S-Aryl Ditihiocarbonates

The reactions of O-ethyl S-phenyl dithiocarbonate (1) and O-ethyl S-(p-nitrophenyl dithiocarbonate (2) with a series of secondary alicyclic amines, namely, piperidine, piperazine, 1-(β-hydroxyethyl)piperazine, morpholine, 1-formylpiperazine, and (with 2 only) piperazinium ion, are subject to a kinetic study at several pH values.The reaction leads to the corresponding thiocarbamates and thiophenols (measured as thiophenoxide ion by UV-vis spectrophotometry).Pseudo-first-order rate coefficients (kobsd) are found throughout (amine excess).The kinetics are first order in amine for the reactions of 2.The plots of kobsd vs for the reaction of 1, except with 1-formylpiperazine, are linear, but near the origin they are curved, showing a more complex rate equation.The reaction of 1 with 1-formylpiperazine shows a second-order dependence on the amine.No dependence on pH of the second-order rate constant values is observed.The findings are well-accommodated by a mechanistic model involving reversible nucleophilic attack on the thiocarbonyl group, two tetrahedral intermediates, 3 and 4, and a deprotonation step.The Bronsted-type plots obtained are linear (β1 = 0.22) for the reaction of 1 and curved for 2 (β1 = 0.2 and β2 = 0.8).The Bronsted-type plot obtained with the rate constants for amine expulsion from 3 is linear with β-1 = -0.67 and -0.54 for the reactions of 1 and 2, respectively.