Mechanisms of Acid Decomposition of Dithiocarbamates. 1. Alkyl Dithiocarbamates

Abstract

The acid decomposition of some substituted methyldithiocarbamates was studied in water at 25 °C in the range of H o −5 and pH 5. The pH−rate profiles showed a bell-shaped curve from which were calculated the acid dissociation constants of the free and conjugate acid species and the specific acid catalysis rate constants k H . The Brønsted plot of k H vs p K N, the dissociation constant of the conjugate acid of the parent amine, suggests that the acid cleavage occurs through two mechanisms that depend on the p K N . The plot presents a convex upward curve with a maximum at p K N 9.2, which is consistent with the cleavage of the dithiocarbamate anion through a zwitterion intermediate and two transition states. For p K N < 9.2, the N-protonation is slower than the C−N bond breakdown. Inverse SIE showed that the zwitterion is formed through a late transition state. At p K N > 9.2, the C−N bond breakdown is the slowest step, and according to the inverse SIE, the transition state changes rapidly with the increase of p K N to a late transition state. The plot shows a minimum at p K N ∼10, indicating that a new mechanism emerges at higher values, and it is postulated that it represents a path of intramolecular S to N proton-transfer concerted with the C−N bond breakdown. The thiocarbonyl group acts as a powerful electron-withdrawing group, decreasing the basicity of the nitrogen of the parent amine by 14.1 p K units.