Abstract

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°C, ionic strength 0.2 M (maintained with KCl). Under amine excess, pseudo-first-order rate coefficients (kobsd) are obtained, which are linearly proportional to the free-pyridine concentration. The second-order rate coefficients (kN) 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 ?=1.0. The EDNPTOC pyridinolysis shows a curved Brönsted-type plot with slopes ?1= 0.1 (high pKa], ?2 = 1.0 (low pKa), and pKa o = 6.8 (pKa 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 (k2 step) at low pKa for EDNPTOC (and in the whole pKa range for MNPTOC and ENPTOC), and there is a change to rate-limiting formation of T± (k1 step) at high pKa for EDNPTOC. Comparison of these Brönsted plots among them and with similar ones permits the following conclusions: (i) There is no variation of kN by substitution of methoxy by ethoxy as the nonleaving group of the substrate. (ii) The pKa o value is smaller for the less basic aryloxide nucleofuge due to a larger k25 value. (iii) The change of C=S by C=O as the electrophilic center of the substrate results in larger values for both k1 (amine expulsion rate) and k2, and also a larger k-1/k2 ratio for the carbonyl derivative. There is also an increase of k1 by the same change. The K1k2 (= k1k2/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 k1 step or the k2 step is rate limiting. This is explained by the softer nature of pyridines than alicyclic amines (k1 step) and the greater nucleofugality (k-1) of the latter amines than isobasic pyridines, leading to a larger k2/k-1 ratio for pyridines (k2 is little affected by the amine nature), and therefore a larger K1k2 value when the k2 step is rate determining.