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 Ho −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 kH. The Brønsted plot of kH vs pKN, the dissociation constant of the conjugate acid of the parent amine, suggests that the acid cleavage occurs through two mechanisms that depend on the pKN. The plot presents a convex upward curve with a maximum at pKN 9.2, which is consistent with the cleavage of the dithiocarbamate anion through a zwitterion intermediate and two transition states. For pKN < 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 pKN > 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 pKN to a late transition state. The plot shows a minimum at pKN ∼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 pK units.