Abstract

The reactions of quinuclidines with ethyl S-(4-nitrophenyl) thiolcarbonate (NPTC), ethyl S-(2,4-dinitrophenyl) thiolcarbonate (DNPTC), and ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate (TNPTC) are subjected to a kinetic study in aqueous solution, 25.0°C, ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under quinuclidine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [N] (N is the free substituted quinuclidine) are linear and pH independent, with slope k(N). The Bronsted-type plots (log k(N) vs pK(a) of quinuclidinium ions) are linear, with slope ? = 0.85 for NPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T±) is rate determining, and ? = 0.54 and 0.47 for DNPTC and TNPTC, respectively, consistent with a concerted mechanism. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) Substitution of the 4-nitrobenzenethio group in T± by 2,4-dinitrobenzenethio or 2,4,6- trinitrobenzenethio destabilizes the tetrahedral intermediate. (ii) Quinuclidines destabilize the tetrahedral intermediate relative to secondary alicyclic amines, anilines, and pyridines. The leaving abilities of isobasic amines from T± follow the sequence pyridines < anilines < secondary alicyclic amines < quinuclidines. (iii) Quinuclidines are more reactive toward the carbonyl group of phenyl 4-nitrophenyl carbonate than that of NPTC.