| Abstract
| - The acid decomposition of some p-substituted aryldithiocarbamates (arylDTCs) was observed in20% aqueous ethanol at 25 °C, μ = 1.0 (KCl, for pH > 0). The pH−rate profiles showed a dumbellshape with a plateau where the observed first-order rate constant kobs was equal to ko, the rateconstant of the decomposition of the dithiocarbamic acid species. The acid dissociation constants ofthe dithiocarbamic acids (pKa) and their conjugate acids (pK+) were calculated from the pH−rateprofiles. Comparatively, ko was more than 104-fold faster than alkyldithiocarbamates (alkDTCs)with similar pKN (the acid dissociation constant of the parent amine). It was observed that thevalues of pKa and pK+ were 5 and 8 units of pK, respectively, higher than the expected values fromthe pKN of alkylDTCs. The higher values were attributed to the inhibition of the delocalization ofthe nitrogen electron pair into the benzene ring because of the strong electron withdrawal effect ofthe thiocarbonyl group. Comparison of the activation parameters showed that the rate accelerationwas due to a decrease in the enthalpy of activation. Proton inventory indicated the existence of amultiproton transition state, and it was consistent with an S to N proton transfer through a watermolecule. There are two hydrogens contributing to a secondary SIE, and there are also two protonsthat are being transferred at the transition state to form a zwitterion followed by fast C−N bondcleavage. The mechanism could also be a concerted asynchronic process where the N-protonationis more advanced than the C−N bond breakdown. The kinetic barrier is similar to the torsionalbarrier of thioamides, suggesting that the driving force to reach the transition state is the neededtorsion of the C−N bond that inhibits the resonance with the thiocarbonyl group and the aromaticmoiety, increasing the basicity of the nitrogen and making the proton transfer thermodynamicallyfavorable.
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