Mechanisms of Acid Decomposition of Dithiocarbamates. 3. Aryldithiocarbamates and the Torsional Effect

Abstract

The acid decomposition of some p-substituted aryldithiocarbamates (arylDTCs) was observed in 20% aqueous ethanol at 25 degrees C, mu = 1.0 (KCl, for pH > 0). The pH-rate profiles showed a dumbell shape with a plateau where the observed first-order rate constant k(obs) was equal to k(o), the rate constant of the decomposition of the dithiocarbamic acid species. The acid dissociation constants of the dithiocarbamic acids (pK(a)) and their conjugate acids (pK(+)) were calculated from the pH-rate profiles. Comparatively, k(o) was more than 10(4)-fold faster than alkyldithiocarbamates (alkDTCs) with similar pK(N) (the acid dissociation constant of the parent amine). It was observed that the values of pK(a) and pK(+)were 5 and 8 units of pK, respectively, higher than the expected values from the pK(N) of alkylDTCs. The higher values were attributed to the inhibition of the delocalization of the nitrogen electron pair into the benzene ring because of the strong electron withdrawal effect of the thiocarbonyl group. Comparison of the activation parameters showed that the rate acceleration was due to a decrease in the enthalpy of activation. Proton inventory indicated the existence of a multiproton transition state, and it was consistent with an S to N proton transfer through a water molecule. There are two hydrogens contributing to a secondary SIE, and there are also two protons that are being transferred at the transition state to form a zwitterion followed by fast C-N bond cleavage. The mechanism could also be a concerted asynchronic process where the N-protonation is more advanced than the C-N bond breakdown. The kinetic barrier is similar to the torsional barrier of thioamides, suggesting that the driving force to reach the transition state is the needed torsion of the C-N bond that inhibits the resonance with the thiocarbonyl group and the aromatic moiety, increasing the basicity of the nitrogen and making the proton transfer thermodynamically favorable.

The mechanisms of hydrolysis of N‐alkyl O‐arylthioncarbamate esters

“…The inverse solvent isotope effect for general base catalysis is consistent with a partial transfer of the proton at a rather late transition state as was suggested by the Brønsted plot and it is expected for a transfer between N to (or from) O 55–58. The water catalyzed rate constant in protium oxide of EMeOT, k N = k o , exhibits an inverse SIE $k_o^D /k_o^H $ = 2.76.…”

Phenyldithiocarbamate Ligands Decompose During Nanocrystal Ligand Exchange

“…Although the ligand is stable for multiple months when stored dry on a shelf, TEA-DMPTC decomposes in solution. Dithiocarbamate decomposition is known to occur through complex multistep reactions that involve the protonation of a sulfur atom. − The triethylammonium deprotonates in solution, causing the H NMR peaks associated with TEA to shift upfield over time (see Figure B). At the same time, the 3,5-dithiocarbamate anion breaks into carbon disulfide and 3,5-dimethylaniline (DMA).…”

Resonance Raman Investigation of the Interaction between Aromatic Dithiocarbamate Ligands and CdSe Quantum Dots

“…One batch of these particles was treated with the DTC ligand. The ligand used in this study was p-methoxyphenyldithiocarbamate (MPDTC, Structure 1), which was synthesized as its ammonium salt using a slight variation of the methods described in the literature 5,27 and detailed in the dried MeOH at a concentration of about 10 mM and the QD-coated coverslips were soaked in this solution for one to two days in the dark at room temperature (~20 °C). Other batches were treated with MPDTC in the same manner but with varying concentrations of cadmium acetate added to provide a source of additional cadmium.…”