Kinetics and mechanism of the pyridinolysis of alkyl aryl thionocarbonates

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 Bronsted-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 Bronsted-type plot with slopes beta(1) = 0.1 (high pK(a)), beta(2) = 1.0 (low pK(a)), and pK(a)(0) = 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 Bronsted 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)(0) value is smaller far 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 of 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.