Phenolysis and Aminolysis of 4-Nitrophenyl and 2,4-Dinitrophenyl S-Methyl Thiocarbonates in Aqueous Ethanol

The reactions of S-methyl O-(4-nitrophenyl) thiocarbonate (1) and S-methyl O-(2,4-dinitrophenyl) thiocarbonate (2) with a series of secondary alicyclic (SA) amines and phenols are subjected to a kinetic investigation. Under nucleophile excess, pseudo-first-order rate coefficients (k(obs)) are obtained. Plots of k(obs) against the free nucleophile concentration at constant pH are linear with slopes k(N). The Bronsted plots (log k(N) vs. nucleophile pK(a)) for the reactions are linear with slope (beta) values in the 0.5-0.7 range, in accordance with concerted mechanisms. Comparison of the SA aminolysis of 1 with the same one carried out in water shows that the change of solvent from water to aqueous ethanol destabilizes the zwitterionic tetrahedral intermediate, changing the mechanism from stepwise to concerted. This destabilization is greater than that due to the change from SA amines to quinuclidines. For the phenolysis reactions, the k(N) values in aqueous ethanol are smaller than those for the same reactions in water. Considering that the nucleophile is an anion, this result is unexpected because the anion should be more stabilized in the more polar solvent. This result is explained by the facts that the phenoxide reactant has a negative charge that is delocalized in the aromatic ring and the transition state is highly polar. (C) 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 353-358, 2011