Mechanisms of acid decomposition of dithiocarbamates. 2. Efficiency of theintramolecular general acid catalysis

The acid decomposition of ethylenebis(dithiocarbamate) (EbisDTC) and glycin edithiocarboxylate (glyDTC) was studied in water at 25 degrees C in the ran ge of H-o -5 to pH 5. The acid dissociation constants of all species involv ed were calculated from LFER and from the pH-rate profiles. According to th e pK(a) of the parent amine of the reactive species, both compounds decompo se through the dithiocarbamate anion and a zwitterion intermediate. The int ermolecular 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 specie s also cleaves through an intramolecular general acid-catalyzed mechanism w here 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 effectiv e molarity is (5.6 +/- 3.3) x 10(2) M. The acid decomposition of the dithio carbamic conjugate acid of EbisDTC anion proceeds through a fast N-protonat ion and a slower C-N breakdown. The intramolecular general acid catalysis r ate constant is (8.2 +/- 2.8) x 10(6) s(-1), but the efficiency of this fas t proton transfer is only (14.3 +/- 4.9) M. The intramolecular general acid catalysis of the free acid forms of the carboxylic and dithiocarbamic grou ps is unfavorable for about 4 kcal mol(-1) with respect to the protonation of the external hydron, and consequently, no external buffer catalysis is e xpected to be observed for dithiocarbamates that decompose through a zwitte rion 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. Esti mation of the strength of the hydrogen bonding in the reagent and product s upports the assumption that a thermodynamically favorable change of hydroge n bonding from reagent to product increases the efficiency of proton transf er.