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 degrees 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) x 10(3) s(-)(1) and the efficiency of the proton-transfer step as measured by the effective molarity is (5.6 +/- 3.3) x 10(2) 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) x 10(6) 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.
Humeres
Eduardo
E
Department of Chemistry, Universidade Federal de Santa Catarina, 88040-900 Florianopolis-SC, Brazil.
Debacher
Nito A.
NA
Sierra
M. Marta de S.
MM