RESUMO
Graphite microparticles (d50 6.20 µm) were oxidized by strong acids, and the resultant graphite oxide was thermally exfoliated to graphene oxide sheets (MPGO, C/O 1.53). Graphene oxide was treated with nonthermal plasma under a SO2 atmosphere at room temperature. The XPS spectrum showed that SO2 was inserted only as the oxidized intermediate at 168.7 eV in the S 2p region. Short thermal shocks at 600 and 400 °C, under an Ar atmosphere, produced reduced sulfur and carbon dioxide as shown by the XPS spectrum and TGA analysis coupled to FTIR. MPGO was also submitted to thermal reaction with SO2 at 630 °C, and the XPS spectrum in the S 2p region at 164.0 eV showed that this time only the nonoxidized episulfide intermediate was inserted. Plasma and thermal treatment produced a partial reduction of MPGO. The sequence of thermal reaction followed by plasma treatment inserted both sulfur intermediates. Because oxidized and nonoxidized intermediates have different reactivities, this selective insertion would allow the addition of selective types of organic fragments to the surface of graphene oxide.
RESUMO
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.
RESUMO
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.
