Abstract

The reactions of secondary alicyclic amines (saa) with ethyl S-4-X- phenyl thiolcarbonates (X = Cl, H, Me, and MeO) are subjected to a kinetic analysis in water, 25.0 °C, ionic strength 0.2 (KCl). By following the leaving groups spectrophotometrically (260-270 nm), under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. Plots of k(obsd) against free-amine concentration at constant pH are linear, with slope (k(N)) independent of pH. The Bronsted-type plots (log k(N) against amine pK(a)) are linear for the aminolysis of the four substrates, with slopes ?(N) = 0.7- 0.8. The magnitudes of the slopes are consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T(±)) whose breakdown to products is rate-determining (k2 step). This mechanism is simpler than that for the same aminolysis of the corresponding ethyl S-aryl dithiocarbonates, which includes proton transfer from T(±) to the amine (k3 step), due to the fact that for the title reactions k2 > k3[amine]. With the results of the present work and those for the reactions of saa with ethyl S-4-nitrophenyl thiolcarbonate, a dual parametric equation is deduced for k(N) as a function of amine and leaving group basicity, with both ?(N) and -?(lg) = 0.8. Another dual parametric equation is deduced from literature data for the pyridinolysis of ethyl S-aryl thiolcarbonates. Comparison of both multiparametric expressions shows that pyridines are more reactive than isobasic saa toward thiolcarbonates when breakdown of T(±) to products is rate-limiting.