The reactions of methyl 4-nitrophenyl, ethyl 4-nitrophenyl, and ethyl 2,4-dinitrophenyl thionocarbonates (MNPTOC, ENPTOC, and EDNPTOC, respectively) with a series of 3- and 4-substituted pyridines are subjected to a kinetic investigation in water, 25.0 degrees C, ionic strength 0.2 M (maintained with KCl). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained, which are linearly proportional to the free-pyridine concentration. The second-order rate coefficients (k(N)) are obtained as the slopes of these plots. The Brönsted-type plots found for the two mononitro derivatives coincide in one straight line (same slope and intercept) of slope beta = 1.0. The EDNPTOC pyridinolysis shows a curved Brönsted-type plot with slopes beta(1) = 0.1 (high pK(a)), beta(2) = 1.0 (low pK(a)), and pK(a)(o) = 6.8 (pK(a) value at the center of curvature). These plots are consistent with the existence of a zwitterionic tetrahedral intermediate (T(+/-)) on the reaction pathway whereby expulsion of aryloxide anion from T(+/-) is rate determining (k(2) step) at low pK(a) for EDNPTOC (and in the whole pK(a) range for MNPTOC and ENPTOC), and there is a change to rate-limiting formation of T(+/-) (k(1) step) at high pK(a) for EDNPTOC. Comparison of these Brönsted plots among them and with similar ones permits the following conclusions: (i) There is no variation of k(N) by substitution of methoxy by ethoxy as the nonleaving group of the substrate. (ii) The pK(a)(o) value is smaller for the less basic aryloxide nucleofuge due to a larger k(2) value. (iii) The change of C=S by C=O as the electrophilic center of the substrate results in larger values for both k(-)(1) (amine expulsion rate) and k(2), and also a larger k(-)(1)/k(2) ratio for the carbonyl derivative. There is also an increase of k(1) by the same change. The K(1)k(2) (= k(1)k(2)/k(-)(1)) values are larger for the pyridinolysis of methyl 2,4-dinitrophenyl and methyl 4-nitrophenyl carbonates compared to the corresponding thionocarbonates (EDNPTOC and MNPTOC, respectively). (iv) Pyridines are more reactive than isobasic secondary alicyclic amines toward ENPTOC when either the k(1) step or the k(2) step is rate limiting. This is explained by the softer nature of pyridines than alicyclic amines (k(1) step) and the greater nucleofugality (k(-)(1)) of the latter amines than isobasic pyridines, leading to a larger k(2)/k(-)(1) ratio for pyridines (k(2) is little affected by the amine nature), and therefore a larger K(1)k(2) value when the k(2) step is rate determining.

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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 (k obsd) 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 pK a of T± and the rate coefficient for proton transfer (k 3 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 Brönsted type plots for amine basicity have slopes βN ca. 0.2 for rate-determining amine attack (k 1) 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 (k 2) from T± (which competes with amine deprotonation of T±) relative to the same expulsion from the analogous oxy intermediate.

The reactions of the title substrate with a series of secondary alicyclic amines are subjected to a kinetic study in 44 wt% aqueous ethanol, 25.0°C, and ionic strength 0.2 M (KC1). The Bronsted-type plot (log k~ vs. pKa of the amine, where k~ is the second-order rate coefficient) obtained is linear with slope p = 0.53, which indicates a concerted mechanism. The predicted Bronsted break for a hypothetical stepwise mechanism is pK,O = 8.7, which was not observed (pKa range of amines: 6-11). The same reaction in water is stepwise, which shows that the tetrahedral intermediate found in water is much destabilized by the change of solvent from water to aqueous ethanol.

The reactions of a series of secondary alicyclic amines with the title substrates are subjected to a kinetic investigation in aqueous solution, 25 degrees C, ionic strength 0.2 M (maintained with KCl). Under amine excess pseudo-first-order rate coefficients (k(obsd)) are found. The plots of k(obsd) against concentration of free amine at constant pH are linear, with the slopes (k(N)) independent of pH. The Brönsted-type plots obtained (log k(N) vs amine pK(a)) for the aminolysis of both substrates are linear with the same slope, beta = 0.26. From this value, the kinetic law, and the analysis of products, it is deduced that these reactions proceed through a zwitterionic tetrahedral addition intermediate (T(+/-)) on the reaction path, and its formation is the rate-determining step. From a comparison of the present reactions with the concerted aminolysis of substituted phenyl chloroformates in acetonitrile it is inferred that the change of S(-) by O(-) in T(+/-) and that of water by acetonitrile as solvent destabilizes T(+/-) in such a way that the stepwise reaction becomes enforced concerted.