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

Abstract

AbstractThe hydrolysis of N‐ethyl O‐p‐methoxyphenylthioncarbamate (EMeOT) at 50 °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 Brønsted β = 0.63 ± 0.07. Water acted as a general base catalyst with (pseudo‐) first‐order rate constant kN = (2.6 ± 0.2) × 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 © 2010 John Wiley & Sons, Ltd.