| Abstract
| - The reactions of methyl 4-nitrophenyl, ethyl4-nitrophenyl, and ethyl 2,4-dinitrophenyl thionocarbonates (MNPTOC, ENPTOC, and EDNPTOC, respectively) with a series of 3-and 4-substitutedpyridines are subjected to a kinetic investigation in water, 25.0 °C,ionic strength 0.2 M (maintainedwith KCl). Under amine excess, pseudo-first-order ratecoefficients (kobsd) are obtained, whicharelinearly proportional to the free-pyridine concentration. Thesecond-order rate coefficients (kN)are obtained as the slopes of these plots. The Brönsted-typeplots found for the two mononitroderivatives coincide in one straight line (same slope and intercept) ofslope β = 1.0. The EDNPTOCpyridinolysis shows a curved Brönsted-type plot with slopesβ1 = 0.1 (high pKa),β2 = 1.0 (low pKa),and pKao = 6.8(pKa value at the center of curvature).These plots are consistent with the existenceof a zwitterionic tetrahedral intermediate (T±) on thereaction pathway whereby expulsion ofaryloxide anion from T± is rate determining(k2 step) at low pKa forEDNPTOC (and in the wholepKa range for MNPTOC and ENPTOC), and there is achange to rate-limiting formation of T±(k1step) at high pKa for EDNPTOC. Comparisonof these Brönsted plots among them and with similarones permits the following conclusions: (i) There is no variation ofkN by substitution of methoxyby ethoxy as the nonleaving group of the substrate. (ii) ThepKao value is smaller for the lessbasicaryloxide nucleofuge due to a larger k2 value.(iii) The change of CS by CO as theelectrophiliccenter of the substrate results in larger values for bothk-1 (amine expulsion rate) andk2, and alsoa largerk-1/k2ratio for the carbonyl derivative. There is also an increase ofk1 by the same change.The K1k2 (=k1k2/k-1)values are larger for the pyridinolysis of methyl 2,4-dinitrophenyl andmethyl4-nitrophenyl carbonates compared to the corresponding thionocarbonates(EDNPTOC andMNPTOC, respectively). (iv) Pyridines are more reactive thanisobasic secondary alicyclic aminestoward ENPTOC when either the k1 step or thek2 step is rate limiting. This is explainedby thesofter nature of pyridines than alicyclic amines(k1 step) and the greater nucleofugality(k-1) of thelatter amines than isobasic pyridines, leading to a largerk2/k-1ratio for pyridines (k2 is littleaffectedby the amine nature), and therefore a largerK1k2 value when thek2 step is rate determining.
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