RESUMEN

Herein, the reactivity and selectivity of the reaction of O,O-diethyl 4-nitrophenyl phosphate triester (Paraxon, 1) with piperidine in ionic liquids (ILs), three conventional organic solvents (COS), and water is studied by (31)P NMR, UV-vis, and GC/MS. Three phosphorylated products are identified as follows: O,O-diethyl piperidinophosphate diester (2), O,O-diethyl phosphate (3), and O-ethyl 4-nitrophenyl phosphate diester (4). Compound 4 also reacts with piperidine to yield O-ethyl piperidinophosphate monoester (5). The results show that both the rate and products distribution of this reaction depend on peculiar features of ILs as reaction media and the polarity of COS.

Asunto(s)

RESUMEN

Preferential solvation in aromatic nucleophilic substitution reactions is discussed using a kinetic study complemented with quantum chemical calculations. The model system is the reaction of a series of secondary alicyclic amines toward phenyl 2,4,6-trinitrophenyl ether in aqueous ethanol mixtures of different compositions. From solvent effect studies, it is found that only piperidine is sensitive to solvation effects, a result that may be traced to the polarity of the solvent composition in the ethanol/water mixture, which points to a specific electrophilic solvation in the aqueous phase.

RESUMEN

The reactions of S-phenyl, S-(4-chlorophenyl), and S-(2,3,4,5,6-pentafluorophenyl) 4-nitrophenyl thiocarbonates (9, 11, and 16, respectively) with a series of secondary alicyclic (SA) amines and those of S-(4-methylphenyl) 4-nitrophenyl thiocarbonate (8) and compounds 9 and 11 with a series of phenols are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically. Under nucleophile excess, pseudo-first-order rate coefficients (k(obsd)) were found. For all these reactions, plots of k(obsd) vs. free amine or phenoxide anion concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Brønsted-type plots (log k(N) vs. pK(a) of the conjugate acids of the nucleophiles) for the aminolysis of 9, 11, and 16 are linear with slopes beta = 0.85, 0.90, and 0.67, respectively. The two former slopes are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate, which breaking to products is rate determining. The latter beta value is consistent with a concerted mechanism. The Brønsted-type plots for the phenolysis of thiocarbonates 8, 9, and 11 are linear with slopes beta = 0.62, 0.70, and 0.69, respectively. These beta values and the absence of curvature at pK(a) = 7.5 confirm a concerted mechanism. In all these reactions, except those of 16, the main nucleofuge is 4-nitrophenoxide, being the thio benzenethiolate the minor nucleofuge. For the reactions of thiocarbonate 16 the main nucleofuge is pentafluorobenzenethiolate whereas little 4-nitrophenoxide was found. The reactions of two SA amines with S-(3-chlorophenyl) 4-nitrophenyl thiocarbonate (10) were subjected to product analysis, showing 60% 4-nitrophenoxide and 40% 3-chlorobenzenethiolate. The study is completed with a theoretical analysis based on the group electrophilicity index, a reactivity descriptor that may be taken as a measure of the ability of a group or fragment to depart from a molecule with the bonding electron pair. The theoretical analysis is in accordance with the experimental results obtained and predicts relative nucleofugalities of O-aryl vs. S-aryl groups in a series of diaryl thiocarbonates not experimentally evaluated to date.

Asunto(s)

RESUMEN

The Brønsted plots for the title reactions are linear with slopes of 0.53-0.56. The magnitude of the slopes and the fact that there are no breaks at the predicted pK(a) for stepwise mechanisms indicate that these reactions are concerted. This finding is in great contrast to the stepwise mechanisms found for the pyridinolysis of other carbonates. The concerted mechanism is attributed to the fact that the title carbonates possess two O-aryl groups, one of them being an exceptionally good nucleofuge.

RESUMEN

[Chemical reaction: See text] The reactions of anilines with 4-nitrophenyl, 4-methylphenyl, and 4-chlorophenyl 4-nitrophenyl carbonates (BNPC, MPNPC and ClPNPC, respectively) are studied kinetically in 44 wt % ethanol-water, at 25.0 degrees C, with an ionic strength of 0.2 M (KCl). Plots of k(obsd) vs [amine] are linear, with the slopes (kN) independent of pH. The Brønsted-type plots (log k(N) vs pKa of conjugate acids of anilines) are linear, with slopes beta = 0.65, 0.85, and 0.78 for the reactions of anilines with BNPC, MPNPC, and ClPNPC, respectively. The values of the slopes for the two latter reactions are in accordance with those obtained in stepwise mechanism where breakdown to product of a zwitterionic tetrahedral intermediate is the rate-determining step. On the other hand, the beta value for the reactions of BNPC is at the upper limit of those found for concerted mechanisms. The kinetic results for the reactions of anilines with BNPC correlates well with those for the concerted reactions of the same amines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates: A plot of the calculated log k(N) values (through a multiple parametric equation) vs the experimental log k(N) values is linear with unity slope and zero intercept, which confirms the concerted mechanism for the latter three reactions.

RESUMEN

[reaction: see text] The title reactions, in 44 wt % ethanol-water at 25.0 degrees C, exhibit slightly curved Brønsted-type plots (log kN versus pKa of amines) with slopes beta1 = 0.1-0.44 (at high pKa) and beta2 ca. 0.7 (at low pKa). The magnitude of some of these slopes, together with the fact that the curvature center (pKa(0) = 9.5-10.8) does not change with the electronic effects of the benzoyl substituent, suggests that these reactions are not stepwise, but concerted.

RESUMEN

[reaction: see text] The reactions of S-2,4-dinitrophenyl 4-methyl (1), S-2,4-dinitrophenyl 4-H (2), S-2,4-dinitrophenyl 4-chloro (3), and S-2,4-dinitrophenyl 4-nitro (4) thiobenzoates with a structurally homogeneous series of pyridines are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically (420 nm) by monitoring the appearance of 2,4-dinitrobenzenethiolate anion. Pseudo-first-order rate coefficients (k(obsd)) are obtained for all the reactions, employing excess of amine. The plots of k(obsd) vs [free pyridine] at constant pH are linear with the slopes (k(N)) independent of pH. The Brønsted-type plots (log k(N) vs pK(a) of the conjugate acid of the pyridines) are curved for all the reactions. The Brønsted curves are in accordance with stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T(+/-)), and a change in the rate-limiting step. An equation based on this hypothesis accounts well for the experimental points. The Brønsted lines were calculated with the following parameters: Reactions of thiolbenzoate 1: beta(1) 0.33 (slope at high pK(a)), beta(2) 0.95 (slope at low pK(a)), and pK(a)(0) = 8.5 (pK(a) at the curvature center); thiolbenzoate 2: beta(1) 0.30, beta(2) 0.88, and pK(a)(0) = 8.9; thiolbenzoate 3: beta(1) 0.33, beta(2) 0.89, and pK(a)(0) = 9.5; thiolbenzoate 4: beta(1) 0.21, beta(2) 0.97, and pK(a)(0) = 9.9. The increase of the pK(a)(0) value with the increase of the electron-withdrawing effect of the acyl substituent is explained by the argument that the rate of pyridine expulsion from T(+/-) (k(-)(1)) is favored over that of 2,4-dinitrobenzenethiolate leaving (k(2)), i.e., k(-)(1)/k(2) increases, as the acyl group becomes more electron withdrawing. The pK(a)(0) values for the title reactions are smaller than those for the reactions of the corresponding 4-nitrophenyl 4-substituted thiolbenzoates with the same pyridine series. This is explained by the larger k(2) value for 2,4-dinitrobenzenethiolate leaving from T(+/-) compared with 4-nitrobenzenethiolate, which results in lower k(-)(1)/k(2) ratios for the dinitro derivatives. The pK(a)(0) value obtained for the pyridinolysis of thiolbenzoate 2 (pK(a)(0) = 8.9) is smaller than that found for the same aminolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5). This is attributed to the greater nucleofugality from T(+/-) of 2,4-dinitrobenzenethiolate (pK(a) of conjugate acid 3.4) relative to 2,4-dinitrophenoxide (pK(a) of conjugate acid 4.1). The title reactions are also compared with the aminolysis of similar esters to assess the effect of the amine nature and leaving and acyl groups on the kinetics and mechanism.

RESUMEN

[reaction: see text] The reactions of secondary alicyclic (SA) amines and quinuclidines (QUI) with 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) and those of SA amines with 2,3,4,5,6-pentafluorophenyl S-methyl thiocarbonate (3) are subjected to a kinetic study in aqueous solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions of thiocarbonates 1, 2, and 3 were followed spectrophotometrically at 400, 360, and 220 nm, respectively. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Brønsted-type plots (log kN vs pKa of aminium ions) are linear for all the reactions, with slopes beta = 0.9 for those of 1 with SA amines and QUI, beta = 0.36 and 0.57 for the reactions of 2 with SA amines and QUI, respectively, and beta = 0.39 for the reactions of SA amines with 3. The magnitude of the slopes indicates that both aminolyses of 1 are governed by stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T+/-), where expulsion of the nucleofuge from T+/- is the rate-determining step. The values of the Brønsted slopes found for the aminolyses of thiocarbonates 2 and 3 suggest that these reactions are concerted. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions arise: (i) Thiocarbonate 2 is more reactive than 1 toward the two amine series. (ii) The change of the nonleaving group from MeO in 4-nitrophenyl methyl carbonate to MeS in thiocarbonate 1 results in lower kN values. (iii) The greater reactivity of this carbonate than thiocarbonate 1 is attributed to steric hindrance of the MeS group, compared to MeO toward amine attack. (iv) The change of a pyridine to an isobasic SA amine or QUI destabilizes the T+/- intermediate formed in the aminolyses of 2. (v) The change of 4-nitrophenoxy to 2,3,4,5,6-pentafluorphenoxy or 2,4-dinitrophenoxy as the leaving group destabilizes the tetrahedral intermediate formed in the reactions with SA amines, changing the mechanism from a stepwise process to a concerted reaction.

RESUMEN

Quantitative relationships are reported between the global electrophilicity index and the experimental rate coefficients for the reactions of thiolcarbonates and dithiocarbonates with piperidine. The validated scale of electrophilicity is then used to rationalize the reaction mechanisms of these systems. This scale also makes it possible to predict both rate coefficients and Hammett substituent constants for a series of systems that have not been experimentally evaluated to date.

Asunto(s)

RESUMEN

The reactions of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) with a series of 3- and/or 4-substituted pyridines in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the release of 4-nitrophenoxide (400 nm) or 2,4-dinitrophenoxide (360 nm) anions. Under amine excess, pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsd vs [pyridine] are linear and pH-independent, with slope kN. The Brønsted-type plot (log kN vs pKa of pyridinium ions) for the reactions of 1 is linear, with slope beta = 1.1, in contrast to the plot for the reactions of 2, which is biphasic, with slopes beta1 = 0.25 (high pKa) and beta2 = 0.90 (low pKa) and the curvature center at pKa = p = 7.3. The latter Brønsted plot is consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T+/-) on the reaction path, and a change of the rate-determining step, from breakdown to formation of T+/-, as pyridine basicity increases. For the reactions of 1 the beta value indicates that the mechanism is also stepwise with expulsion of the nucleofuge from T+/- as the rate-determining step. By comparison of the reactions under investigation among each other and with similar aminolyses, the following conclusions can be drawn. (i) Thiocarbonate 2 is more reactive than 1 toward pyridines. (ii) The pka0 value for the pyridinolysis of 2,4-dinitrophenyl methyl carbonate (4) is larger than that for thiocarbonate 2. (iii) The k1 values (pyridine attack to form T+/-) are smaller for thiocarbonates 1 and 2 than the corresponding oxy carbonates 3 and 4, respectively. This is not in accordance with the electronic effects of MeS and MeO and could be attributed to steric hindrance of the MeS group toward pyridine attack. (iv) The kN values for the pyridinolysis of carbonates 3 and 4 are larger than those for thiocarbonates 1 and 2, respectively, when the k2 step is rate-limiting.

RESUMEN

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.

RESUMEN

The title reactions are subjected to a kinetic study in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). By following the reactions spectrophotometrically two consecutive reactions are observed: the first is formation of the corresponding thionocarbamates (1-(aryloxythiocarbonyl)pyridinium cations) and the second is their decomposition to the corresponding phenol and pyridine, and COS. Pseudo-first-order rate coefficients (k(obsd1) and k(obsd2), respectively) are found under excess amine. Plots of k(obsd1) vs free pyridine concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Brønsted-type plots (log k(N) vs pK(a) of the conjugate acids of the pyridines) are linear with slopes beta = 0.07 and 0.11 for the reactions of phenyl and 4-nitrophenyl chlorothionoformates, respectively. These Brønsted slopes are in agreement with those found in other stepwise reactions of the same pyridines in water, where the formation of a tetrahedral intermediate is the rate-determining step. In contrast to the stepwise mechanism of the title reactions that for the reactions of the same substrates with phenols is concerted, which means that substitution of a pyridino moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate. The second reaction corresponds to the pyridine-catalyzed hydrolysis of the corresponding 1-(aryloxythiocarbonyl)pyridinium cation. Plots of k(obsd2) vs free pyridine concentration at constant pH are linear, with the slope (k(H)) independent of pH. The Brønsted plots for k(H) are linear with slopes beta = 0.19 and 0.26 for the reactions of the phenyl and 4-nitrophenyl derivatives, respectively. These low values are explained by the fact that as pK(a) increases the effect of a better pyridine catalyst is compensated by a worse leaving pyridine from the corresponding thionocarbamate

RESUMEN

The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Brönsted-type plots are biphasic, with slopes (at high pK(a)) of beta(1) = 0.20 for the reactions of SA amines with 1 and 2 and beta(1) = 0.10 for the pyridinolysis of 1 and with slopes (at low pK(a)) of beta(2) = 0.80 for the reactions of SA amines with 1 and 2 and beta(2) = 1.0 for the pyridinolysis of 1. The pK(a) values at the curvature center (pK(a)(0)) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T++) and a change in the rate-determining step with the variation of amine basicity. The larger pK(a)(0) value for the pyridinolysis of 1 compared to that for 2 (pK(a)(0) = 6.8) and the larger pK(a)(0) value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pK(a)(0) values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pK(a)(0) value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pK(a)(0) = 9.2) is explained by the greater nucleofugality from T(++) of 2,4-dinitrophenoxide (DNPO(-)) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T(++) by the change of O(-) to S(-). The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T(++)) is in contrast to the more complex mechanism (two tetrahedral intermediates, T(++) and T(-), the latter formed by deprotonation of T(++) by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO(-)) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T by replacement of NPO(-) with DNPO(-) as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T(++) of DNPO(-) than NPO(-) and (ii) the similar rates of deprotonation of both T(++) (formed with DNPO and NPO).

RESUMEN

Reactions of O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC), O-ethyl 2,4,6-trinitrophenyl dithiocarbonate (ETNPDTC), and O-methyl O-(2,4-dinitrophenyl) thiocarbonate (MDNPTOC) with a series of benzenethiolate anions in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. Under excess benzenethiolate, these reactions obey pseudo-first-order kinetics and are first order in benzenethiolate. Nonetheless, similar reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with the ethyl trinitrophenyl ester (ETNPDTC), which showed overall second-order kinetics. The nucleophilic rate constants (k(N)) are pH independent, except those for the reactions of ETNPDTC with the X-benzenethiolates with X = H, 4-Cl, and 3-Cl, which increase as pH decreases. The Brønsted-type plots (log k(N) vs pK(a) of benzenethiols) are linear with slopes beta = 0.66 for the reactions of both ethyl dinitrophenyl ester (EDNPDTC) and ethyl trinitrophenyl ester (ETNPDTC) and beta = 0.58 for those of the thiocarbonate ester (MDNPTOC). For the benzenethiolysis of MDNPTOC and EDNPDTC, no breaks were found in the Brønsted-type plots at pK(a) 4.1 and 3.4, respectively, consistent with concerted mechanisms. Benzenethiolysis of the ethyl trinitrophenyl ester (ETNPDTC) should also be concerted in view of the even more unstable tetrahedral "intermediate" that would have been formed had this reaction been stepwise. ETNPDTC is more reactive toward benzenethiolate anions than EDNPDTC due to the better leaving group involved in the former substrate. The k(N) values found for the reactions of EDNPDTC with benzenethiolates are larger than those obtained for the concerted reactions of the same substrate with isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle. The concerted mechanism for the benzenethiolysis of MDNPTOC, in contrast to the stepwise mechanism found for the phenolysis of this substrate, is attributed to the greater kinetic instability of the hypothetical tetrahedral "intermediate" formed in the former reaction, due to the greater nucleofugality of ArS(-) compared with an isobasic ArO(-). Benzenethiolates are more reactive toward MDNPTOC and EDNPDTC than the corresponding carbonate and thiolcarbonate, respectively. This is also in accordance with the HSAB principle, since benzenthiolates are relatively soft bases that prefer to bind to a relatively soft thiocarbonyl center rather than a relatively hard carbonyl center.

RESUMEN

The reactions of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO(2): 1, 2, and 3, respectively) with a series of secondary alicyclic amines (SAA) were subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions were followed spectrophotometrically by monitoring the release of 4-nitrobenzenethiolate anion at 420-425 nm. Under excess amine, pseudo-first-order rate constants (k(obsd)) are obtained for all reactions. The plots of k(obsd) vs [SAA] at constant pH are linear with the slope (k(N)) independent of pH. The statistically corrected Brønsted-type plots (log k(N)/q vs pK(a) + log p/q) for the reactions of 1 and 2 are nonlinear with slopes at high pK(a), beta(1) = 0.27 and 0.10, respectively, and slopes at low pK(a), beta(2) = 0.86 and 0.84, respectively. The Brønsted curvature is centered at pK(a) (pK(a)(0)) 10.0 and 10.4, respectively. The reactions of SAA with 3 exhibit a linear Brønsted-type plot of slope 0.81. These results are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T(+/-)). For the reactions of 1 and 2, there is a change in rate-determining step with amine basicity, from T(+/-) breakdown to products at low pK(a), to T(+/-) formation at high pK(a). For the reactions of 3, breakdown to products of T(+/-) is rate limiting for all the SAA series (pK(a)(0) > 11). The increasing pK(a)(0) value as the substituent in the acyl group becomes more electron withdrawing is attributed to an increasing nucleofugality of SAA from T(+/-). The greater pK(a)(0) value for the reactions of SAA with 1, relative to that found in the pyridinolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5), is explained by the greater nucleofugality from T(+/-) of the former amines, compared to isobasic pyridines, and the greater leaving ability from T(+/-) of 2,4-dinitrophenoxide relative to 4-nitrobenzenethiolate.

RESUMEN

The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC), O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC) are studied kinetically in water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate, and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Brønsted-type plot of slope beta = 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of MDNPTOC and PDNPTOC exhibit linear Brønsted-type plots of slopes beta = 0.27 and 0.28, respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral intermediate (T(-)) is rate determining. By comparison of the kinetics and mechanisms of the reactions under investigation with similar reactions, the following conclusions arise: (i). Substitution of S(-) by O(-) in the intermediate T(-) destabilizes this species. (ii). The change of DNPO in T(-) to DNPS also destabilizes this intermediate. (iii). Substitution of MeO by PhO as the nonleaving group of the substrate does not affect the kinetics, probably by a compensation of electronic and steric effects. (iv). The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes the intermediate.

RESUMEN

The reactions of anilines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates (MPDNPC and ClPDNPC, respectively) and the latter substrate with secondary alicyclic (SA) amines are subjected to a kinetic study in 44 wt % ethanol-water solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of 2,4-dinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [amine] are linear and pH-independent, with slope k(N). The Brønsted-type plots (log k(N) vs pK(a) of aminium ions) are linear, with slopes beta = 0.68 and 0.66 for the reactions of anilines with MPDNPC and ClPDNPC, respectively, and beta = 0.44 for the reactions of SA amines with ClPDNPC. The magnitude of the slope for the latter reaction indicates that its mechanism is concerted. The slope values for the reactions of anilines are in the borderline between stepwise and concerted mechanisms. The sensitivity of logk(N) to the basicity of the nonleaving group (beta(nlg)) is ca. -0.7 for the reactions of anilines, in agreement with that found for the SA reactions (beta(nlg) ca. -0.6). These results suggest that the reactions of anilines are concerted, although it is also possible that both mechanisms (stepwise and concerted) operate simultaneously. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions can be drawn: (i) ClPDNPC is more reactive than MPDNPC toward the two amine series. (ii) The change of water to aqueous ethanol destabilizes a zwitterionic tetrahedral intermediate. (iii) The change of the nonleaving group from MeO to 4-methylphenoxy or 4-chlorophenoxy also destabilizes this intermediate.

RESUMEN

Reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitrophenyl carbonate (ClPNPC) with a series of quinuclidines (QUIN) and the latter carbonate with a series of secondary alicyclic amines (SAA) are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically at 330 or 400 nm (4-nitrophenol or 4-nitrophenoxide anion appearance, respectively). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are found. For all these reactions, plots of k(obsd) vs free amine concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Brönsted-type plots (log k(N) vs pK(a) of the conjugate acids of the amines) for the reactions of the series of QUIN with MPNPC and ClPNPC are linear with slopes (beta(N)) 0.88 and 0.87, respectively, which are explained by a stepwise process where breakdown of a zwitterionic tetrahedral intermediate (T(+/-)) to products is rate limiting. The Brönsted-type plot for the reactions of the series of SAA with ClPNPC is biphasic with slopes beta(1) = 0.2 (high pK(a) region) and beta(2) = 0.9 (low pK(a) region) and a curvature center at pK(a)(0) = 10.6. This plot is in accordance with a stepwise mechanism through T(+/-) and a change in the rate-determining step, from T(+/-) breakdown to T(+/-) formation as the basicity of the SAA increases. Two conclusions arise from these results: (i) QUIN are better leaving groups from T(+/-) than isobasic SAA, and (ii) the non-leaving group effect on k(N) for these reactions is small, since beta(nlg) ranges from -0.2 to - 0.3. From these values, it is deduced that ClPNPC is ca. 70% more reactive than MPNPC toward SAA and QUIN, when expulsion of the leaving group from T(+/-) is the rate determining step.

RESUMEN

The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Brønsted-type plots for the nucleophilic rate constants are linear with slopes beta = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Brønsted plot was found for the reactions of DNPC and DNPTC at pK(a) ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle.

RESUMEN

The reactions of methyl 4-nitrophenyl carbonate (MNPC) with a series of secondary alicyclic amines (SAA) and quinuclidines (QUIN), methyl 2,4-dinitrophenyl carbonate (MDNPC) with QUIN and 1-(2-hydroxyethyl)piperazinium ion (HPA), and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with SAA are subjected to a kinetic investigation in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M. By following spectrophotometrically the nucleofuge release (330-400 nm) under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. Plots of k(obsd) vs [amine] at constant pH are linear, with the slope (k(N)) being pH independent. The Brönsted-type plot (log k(N) vs amine pK(a)) for the reactions of SAA with MNPC is biphasic with slopes beta(1) = 0.3 (high pK(a) region) and beta(2) = 1.0 (low pK(a) region) and a curvature center at pK(a)(0) = 9.3. This plot is consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T(+/-)) and a change in the rate-determining step with SAA basicity. The Brönsted plot for the quinuclidinolysis of MNPC is linear with slope beta(N) = 0.86, in line with a stepwise process where breakdown of T(+/-) to products is rate limiting. A previous work on the reactions of SAA with MDNPC was revised by including the reaction of HPA. The Brönsted plots for the reactions of QUIN and SAA with MDNPC and SAA with PDNPC are linear with slopes beta = 0.51, 0.48, and 0.39, respectively, consistent with concerted mechanisms. Since quinuclidines are better leaving groups from T(+/-) than isobasic SAA, yielding a less stable T(+/-), it seems doubtful that the quinuclidinolysis of PDNPC is stepwise, as reported.