The reduction of SO 2 on activated carbon was studied in the range of 600-700 C in a differential reactor under steady-state conditions and under chemically controlled kinetics. Initial rates of carbon conversion and gas reagent were calculated from the mass balance of the gaseous products. The kinetics was first-order with respect to carbon and first-order with respect to the partial pressure of SO 2 . The activation parameters were ÁH 6 ¼ ¼ 21.5 kcal mol À1 and ÁS 6 ¼ ¼ À211 cal mol À1 K À1 . The activated carbon was ca. 10 5 times more reactive than graphite, and determined by the enthalpy of activation. The main reaction products were CO 2 and sulfur. CO and COS were produced from consecutive reactions of the primary products. During the pre-steady state, the sulfur content of the carbon increased to a plateau where the reaction reached the steady state condition. This sulfur was shown to be chemically bound to the carbon matrix and represents the stable reactive intermediates of the reduction of SO 2 . The XPS spectrum of the residual carbon C(S) showed two forms of sulfur bound to carbon: non-oxidized sulfur (sulfide and/or disulfide) and oxidized sulfur (sulfone, sulfoxide, sulfenate, sulfinate). The sulfur intermediates C(S) reacted with SO 2 at the same rate as pure activated carbon and with CO 2 to produce SO 2 by the reverse reaction. The reaction of C(S) with CO produced COS.

The acid decomposition of some substituted methyldithiocarbamates was studied in water at 25 °C in the range of H o −5 and pH 5. The pH−rate profiles showed a bell-shaped curve from which were calculated the acid dissociation constants of the free and conjugate acid species and the specific acid catalysis rate constants k H. The Brønsted plot of k H vs pK N, the dissociation constant of the conjugate acid of the parent amine, suggests that the acid cleavage occurs through two mechanisms that depend on the pK N. The plot presents a convex upward curve with a maximum at pK N 9.2, which is consistent with the cleavage of the dithiocarbamate anion through a zwitterion intermediate and two transition states. For pK N < 9.2, the N-protonation is slower than the C−N bond breakdown. Inverse SIE showed that the zwitterion is formed through a late transition state. At pK N > 9.2, the C−N bond breakdown is the slowest step, and according to the inverse SIE, the transition state changes rapidly with the increase of pK N to a late transition state. The plot shows a minimum at pK N ∼10, indicating that a new mechanism emerges at higher values, and it is postulated that it represents a path of intramolecular S to N proton-transfer concerted with the C−N bond breakdown. The thiocarbonyl group acts as a powerful electron-withdrawing group, decreasing the basicity of the nitrogen of the parent amine by 14.1 pK units.

No abstract

No abstract

The reduction of SO 2 on carbons proceeds through reactive intermediates bound to the carbon matrix, which were postulated to be 1,2-oxathiene 2-oxide (or sultine), and 1,3,2-dioxathiolane that decomposes to produce an episulfide and CO 2 . The reactivity of these intermediates was studied in this work through several reactions, using XPS and NMR spectra to postulate their mechanisms. When modified activated carbon obtained after reaction with SO 2 at 630 °C was heated at 900 °C, it was observed that the changes of the XPS spectrum resulted from the forward reaction of decomposition of the oxidized intermediate with S-transfer to produce the episulfide and CO 2 and the reverse reaction with expulsion of SO 2 . Strong bases hydrolyzed the dioxathiolane intermediate and the episulfide. The thiolysis, aminolysis, and reaction of alkyl halides with modified activated carbon occurred with the insertion of the organic moiety in the carbon matrix. Laser photolysis at 266 nm in t-butanol showed insertion of t-butoxide on the matrix. Consistent mechanisms for these reactions were postulated. These results provide additional evidence on the mechanism of reduction of SO 2 on carbons and the chemical nature of the intermediates, offering a new method to modify the physical and chemical properties of a carbon matrix by functionalization with an organic moiety.

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.

No abstract

dGraphite particles (0.505 mm) were oxidized to graphite oxide with KClO3 in a H 2 SO 4 /HNO 3 mixture. Graphite and graphite oxide particles were modified by reaction with SO 2 at 630°C. Thiolysis with sodium dodecane-1-thiolate and aminolysis with dodecane-1-amine of these particles occurred with the insertion of the organic moiety in the carbon matrix. Graphite microparticles (6.20 μm) were oxidized by H 2 SO 4 / KMnO 4 / H 2 O 2 mixture and were exfoliated to graphene oxide sheets (MPGO). MPGO was modified by reaction with SO 2 at 630°C. The modified MPGO was refluxed in DMSO with dodecane-1-thiol, dodecane-1-amine, and hexadecane-1-bromide. The reactions occurred with the insertion of the organic moiety in the carbon matrix, according to the X-ray photoelectron and nuclear magnetic resonance spectra. Mechanisms for the reactions were postulated using the atom inventory technique. Despite the structural differences, graphite, graphite oxide, and graphene oxide present the same selectivity for aminolysis and thiolysis reactions, with respect to the oxidized and non-oxidized intermediates of the reduction of SO 2 , as was found for the activated carbon.