The mechanisms of hydrolysis of N-alkyl O-arylthioncarbamate esters

The hydrolysis of N-ethyl O-p-methoxyphenylthioncarbamate (EMeOT) at 50 degrees C was studied in the range of H-o(X) -3.6 to H- 13.7. The pH-rate profile showed that the hydrolysis occurred through specific acid catalysis at pH <2, spontaneous hydrolysis at pH 2-3, and specific basic catalysis at pH >3. The excess acidity plot against X was linear with slope 0.93, and the Bunnett-Olsen coefficient was 0.07. The acid hydrolysis occurred by an A1 mechanism. The basic hydrolysis of EMeOT can be explained if the mechanism is E1cb. At pH >3 the rate constants increased, reaching a constant value indicating that the expulsion of the aryloxy is not concerted. The neutral species hydrolyzed with general base catalysis with Bronsted beta = 0.63 +/- 0.07. Water acted as a general base catalyst with (pseudo-) first-order rate constant k(N) = (2.6 +/- 0.2) X 10(-8) s(-1), and inverse kinetic solvent isotope effect of k(o)(D)/k(o)(H) = 2.76 consistent with a transfer of the proton at a late transition state, as also supported by the highly negative entropy of activation (-31 cal K-1 mol(-1)). The polynomial expression of the proton inventory curve presented a minimum of the standard deviations that favors the assumption that there are three active protons. The N to O proton transfer to the water molecule forms an incipient hydron. The fractionation factor of the TS of the protons indicated that the hydron is ca. 68% developed at the transition state. Copyright (C) 2010 John Wiley & Sons, Ltd.