The mechanisms of hydrolysis of alkyl N-alkylthioncarbamate esters at 100 °C

The hydrolysis of ethyl N-ethylthioncarbamate (ETE) at 100 degrees C Was Studied in the range of 7 mol/L HCl to 4 mol/L-NaOH. The pH-rate profile showed that the hydrolysis occurred through specific acid catalysis at pH < 2, spontaneous hydrolysis at pH 2-6.5, and specific basic catalysis at pH > 6.5. The Hammett acidity plot and the excess acidity plot against X were linear. The Bunnett-Olsen plot gave I negative slope indicating that the conjugate acid was less hydrated than the neutral substrate. It was concluded that the acid hydrolysis occurred by an Al mechanism. The neutral species hydrolyzed with general base catalysis shown by the Br phi nsted plot with beta = 0.48 +/- 0.04. Water acted as a general base catalyst with (pseudo-)first-order rate constant, k(N) = 3.06 x 10(-7) s(-1). At pH > 6.5 the rate constants increased, reaching it plateau at high basicity. The basic hydrolysis rate constant of ethyl N,N-diethylthioncarbamate, which must react by a B(Ac)2 mechanism, increased linearly at 1-3 mol/L NaOH with a second-order rate constant, k(2) 2.3 x 10(-4) (mol/L)(-1) s(-1), which was 10 times slower than that expected for ETE. Experiments of ETE in 0.6 mol/L NaOH with all excess of ethylamine led to the formation of diethyl thiourea, presenting strong evidence that the basic hydrolysis occurred by the Elcb mechanism. In file rate-determining step, the Elcb mechanism involved the elimination of ethoxide ion front the thioncarbamate anion, producing all isothiocyanate intermediate that decomposed rapidly to form ethylamine, ethanol, and COS.