Hydrolysis of N‐aryl thioncarbamate esters. Modified Marcus equation for reactions with asymmetric intrinsic barriers

Abstract

AbstractThe hydrolysis of ethyl N‐p‐substituted arylthioncarbamates was studied at 100 °C in the pH range 6.5–12.5. No general catalysis was found, and the presence of an isothiocyanate intermediate was detected, indicating that the alkaline hydrolysis occurs by an E1cb mechanism. From the pH–rate profiles, the first‐order rate constants kE for the elimination step of the thioncarbamate anion forming the isothiocyanate intermediate were determined. The alkaline hydrolysis of p‐substituted arylisothiocyanates was studied at 25 °C in 0.1–0.3 M solutions of NaOH and in 0.1–0.3 M aqueous ethanol solutions, at different concentrations of NaOH. The second‐order rate constants for the addition reaction with hydroxide (kOH) and ethoxide (kA) ions were obtained. Leffler plots for the elimination of the ethoxide ion from the arylthioncarbamate anion and for the addition of the ethoxide ion to the arylisothiocyanate were linear. From Leffler's equation, with the sole condition that dαL/dΔG should be constant, a modified Marcus equation (MME) was obtained, where a parameter p (or q for the reverse reaction) defined the asymmetry of the intrinsic barrier. (When p = 1/2 the barrier is symmetric and the MME becomes the Marcus equation in the usual form.) For the addition–elimination reaction studied, both Leffler plots were adjusted to MME with the asymmetric parameter p = 0.694 ± 0.002 for the addition and q = 0.307 ± 0.002 for the elimination reaction. The intrinsic barrier was $\Delta G_{0}^{\neq} = 24.75 \pm 0.02\,\hbox{kcal mol}^{-1}$ and |ΔGmax| = 438 ± 4 kcal mol−1 (1 kcal = 4.184 kJ). The addition reaction was exoergic and, as expected from the high intrinsic barrier, αL changed very little in the series (0.679–0.683); the transition state was product‐like, and it moved towards the reagents with increasing exoergicity. Copyright © 2002 John Wiley & Sons, Ltd.