Kinetics and mechanism of the reactions of anilines with ethyl S-aryl thiocarbonates

The reactions of anilines with ethyl S-(2,4-dinitrophenyl) thiocarbonate (DNPTC) and ethyl S-(2,4,6-trinitrophenyl) thiocarbonate (TNPTC) are subjected to a kinetic study in aqueous solution at 25.0 degrees C and ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under aniline excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [N] (N is the free substituted aniline) are linear and pH independent, with slope k(N). The Bronsted-type plots (log k(N) vs pK(a) of anilinium ions) are linear, with slopes beta = 0.9 for DNPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T+/-) is rate determining, and beta = 0.54 for TNPTC, consistent with a concerted mechanism. Consideration of the results for aminolysis from the present work and those from previous studies leads to the following conclusions. (i) The tetrahedral intermediate possessing a 2,4-dinitrobenzenethio group is more stable than that including the 2,4,6-trinitrobenzenethio group. (ii) The tetrahedral intermediate possessing the 2,4,6-trinitrobenzenethio group has no existence beyond the limit of a vibration period (concerted mechanism). (iii) Tetrahedral intermediates possessing anilino groups are less stable than those derived from pyridines but are more stable than the tetrahedral intermediates derived from secondary alicyclic amines. (iv) Anilines are more reactive toward the carbonyl group of methyl 2,4-dinitrophenyl carbonate than toward the carbonyl of DNPTC.