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

The reactions of anilines with ethyl S-(2,4-dinitrophenyl) thiocarbonate (D NPTC) and ethyl S-(2,4,6-trinitrophenyl) thiocarbonate (TNPTC) are subjecte d to a kinetic study in aqueous solution at 25.0 degrees C and ionic streng th 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anio n. 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 lin ear 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, i n 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 a minolysis from the present work and those from previous studies leads to th e following conclusions. (i) The tetrahedral intermediate possessing a 2,4- dinitrobenzenethio group is more stable than that including the 2,4,6-trini trobenzenethio group. (ii) The tetrahedral intermediate possessing the 2,4, 6-trinitrobenzenethio group has no existence beyond the limit of a vibratio n period (concerted mechanism). (iii) Tetrahedral intermediates possessing anilino groups are less stable than those derived from pyridines but are mo re stable than the tetrahedral intermediates derived from secondary alicycl ic amines. (iv) Anilines are more reactive toward the carbonyl group of met hyl 2,4-dinitrophenyl carbonate than toward the carbonyl of DNPTC.