Mechanisms of Acid Decomposition of Dithiocarbamates. 2. Efficiency of the Intramolecular General Acid Catalysis

The acid decomposition of ethylenebis(dithiocarbamate) (EbisDTC) and glycinedithiocarboxylate (glyDTC) was studied in water at 25 °C in the range of H o −5 to pH 5. The acid dissociation constants of all species involved were calculated from LFER and from the pH−rate profiles. According to the pK a of the parent amine of the reactive species, both compounds decompose through the dithiocarbamate anion and a zwitterion intermediate. The intermolecular N-protonation rate constant of the carboxylic conjugate acid of glyDTC anion is 12.6 M-1 s-1, slower than the C−N breakdown. This species also cleaves through an intramolecular general acid-catalyzed mechanism where the rate constant for the N-protonation is (7.1 ± 4.2) × 103 s-1 and the efficiency of the proton-transfer step as measured by the effective molarity is (5.6 ± 3.3) × 102 M. The acid decomposition of the dithiocarbamic conjugate acid of EbisDTC anion proceeds through a fast N-protonation and a slower C−N breakdown. The intramolecular general acid catalysis rate constant is (8.2 ± 2.8) × 106 s-1, but the efficiency of this fast proton transfer is only (14.3 ± 4.9) M. The intramolecular general acid catalysis of the free acid forms of the carboxylic and dithiocarbamic groups is unfavorable for about 4 kcal mol-1 with respect to the protonation of the external hydron, and consequently, no external buffer catalysis is expected to be observed for dithiocarbamates that decompose through a zwitterion intermediate. The difference between the pK b of the proton acceptor and the pK a of the donor follows the order of the proton efficiency. Estimation of the strength of the hydrogen bonding in the reagent and product supports the assumption that a thermodynamically favorable change of hydrogen bonding from reagent to product increases the efficiency of proton transfer.