Polysulfide reaction mechanism

Under these circumstances, it seems reasonable to look for alternative explanations for the polysulfide reactions and in this connection it is interesting to note that a bimolecular mechanism is in better agreement with the data reported for the trisulfide disproportionation discussed above. This reaction has been followed by H n.m.r. spectroscopy at different temperatures and initial concentrations. Equimolar amounts of diethyl and di-n-propyl trisulfide were used and after the equilibrium had been established the equilibrium constant... 

A reaction mechanism for the breaking of polysulfide bonds in the presence of propane thiol/pipeiidine probe was proposed by Saville and Watson. The thiol-amine combination gives an associate, possibly pipeiidinium propane-2-thiolate ion pair where sulfur atoms enhance the nucleophilic character that is responsible for cleaving... 

Sedimentary Fe(lll) oxyhydroxides, of which goethite is the most common, are the source of iron for most precipitated Fe(II) sulfides. Pyzik and Sommer (1981) (see also Morse et al. 1987) have proposed a number of possible reaction mechanisms for the reduction of goethite to FeS. All involve initial reactant HS , with production of Fe % and S°, polysulfide ions or thousulfate. These species then react to form the Fe(II) sulfides. Their initiating reactions are... 

Bernard et al. (12, 13) used chronopotentiometry and spectrophotometry to demonstrate that two intermediates, a polysulfide (Sn2 ) and a supersulfide (Sn ), are involved in the sulfide oxidation. A definitive reaction mechanism, however, could not be proposed because the authors stated (12) that their reaction was spontaneous and indicated (13) that they could not identify the oxidizing agent. 

No branching or cross-linking occurs during the condensation with difunctional halides. The additiik of a polyfunctional halide such as 1,2,3-trichloropropane is used to produce a network structure, when desired, in the polysulfide polymers. Copolymers are readily prepared by using a mixture of halides in the initial reaction. It has been recently found that redistribution between disulfide groups occurs readily in the presence of sodium polysulfide. This mechanism is similar in results to ester interchange. A copolymer will, therefore, be a random copol rmer rather than a block copolymer. 

These processes of isotope evolution explain the wide diversity of both 5 8 values for the organic-8 and the pyrite, as well as the fact that the pyritic " 8 values are the most " 8 depleted. Because of the complexity of the various mechanisms controlling the formation of iron sulfide — pyrite (see review by Goldhaber (2004)) and the slower reaction of the polysulfide reaction with the 80M, the difference of " 8 values for pyrite compared to the organic-S varies. Those variations A6 " 8poiysuifides pyrite will later control the A5 " 8organic-pyrite-... 

The strong oxidative agents of polysulfide oligomers also are peracids. In this latter case, reaction mechanism foresees initial formation of ion radicals ... 

Kinetic studies conducted by DeBerry (1989) suggest that the polysulfide formation reaction mechanism can be represented by the following equation ... 

Regardless of the actual mechanism, polysulfides play a major role in the sulfur formation in the Stretford process using pentavalent vanadium as the primary oxidant. A more detailed study of the reaction mechanism and kinetics of Stretford reactions was reported by Ryder and Smith (1962) and Phillips and Fleck (1979). 

The generally accepted reaction mechanism [98] of an accelerated sulfur vulcanization is 1) the accelerator (Ac) reacts with sulfur to give monomeric polysulfides of the structure, Ac-S -Ac where Ac is an organic radical derived from the accelerator 2) the monomeric polysulfides subsequently react with rubber to form polymeric sulfides, rubber-S -Ac, 3) the... 

The above-mentioned method is useful but metals that form strong M-S bonds (e.g., Hg, Ag, Sn) do not dissolve in W-Melm solutions of sulfur. This problem has been solved by the addition of Mg to the reaction mixture. Metal polysulfides having a variety of metals can be synthesized by the 7 T-Melm/ M-i-Mg/Sg method (Scheme 11) [48]. For example, a mixture of Mg, Sb powder (1 eq.), Sg (15 eq. as S) and W-Melm is heated at 80 °C for 48 h to afford the orange powder of [Mg(N-MeIm)5]Sb2Sj ( x 15) in 88% yield. Rauchfuss et al. proposed the mechanism of these reactions as follows. First, the reduction of Sg with Mg occurs to give the [Mg(W-MeIm)6] salt of Sg , which is probably in equilibrium with Sg, Ss ", Ss" and other species. Independently, the sulfuration of the thiophilic metal takes place. Next, the polysulfide an-... 

The mechanisms (a)-(c) can also be discussed in connection with the thermal decomposition and rearrangement reactions of organic polysulfides. For example, dimethyl tetrasulfide when heated to 80 °G for several hours disproportionates to the corresponding tri-, penta- and hexasulfides. On prolonged heating small amounts of disulfide are formed in addition... 

The technically most important polysulfide is poly thiophenylene or poly(p-phe-nylene sulfide), PPS. It is obtained by reacting sodium sulfide and p-dichlo-robenzene in a polar solvent, for example, l-methyl-2-pyrrolidone at about 280 °C under pressure. The mechanism of the reaction is very complex and cannot be described by a simple aromatic substitution. This synthesis requires special autoclaves and is therefore not suitable for a laboratory course (for an experimental procedure see Table 2.3). 

The accelerator first reacts with sulfur to produce a polysulfidic intermediate (33) which can then react with a rubber polymer (PH) to give a crosslink precursor (34 Scheme 4) (74MI11501). The reactions are probably free radical in nature, although ionic mechanisms cannot be excluded. Zinc salts and fatty acids appear to be necessary to initiate vulcanization which becomes autocatalytic. 

This reaction also plays a role in the degradation of polysulfides. A back-biting mechanism as shown in equation 6 results in formation of the cyclic disulfide (5). Steam distillation of polysulfides results in continuous gradual collection of (5). There is an equilibrium between the linear polysulfide polymer and the cyclic disulfide. Although the linear polymer is favored and only small amounts of the cyclic compound are normally present, conditions such as steam distillation, which remove (5), drive the equilibrium process toward depolymerization. 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Mechanisms of action for the metal centers in acetylene hydratase, polysulfide reductase, and formate dehydrogenase have been briefly described in Sections VI.A and VLB. The discussion, in each case, was relatively straightforward insofar as the natures of these reactions lend themselves to simple mechanistic proposals. The mechanism by which the metal centers function in most of the other Mo and W enzymes is not as obvious. We elect to discuss mechanistic roles for the molybdenum centers in xanthine oxidase, sulfite oxidase, and dmso reductase. These enzymes are representative members of each large class of molybdenum enzymes, and the large body of literature on each enzyme makes detailed discussion possible. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

The rate constants kt and k2 were found to be 0.5 M 1 min-1 and 29.0 M-1 min-1 respectively at pH 5.05 and 25 C. The rate law and other data suggest a nucleophilic displacement by the bisulfide ion (HS ) on H202 as the rate-determining step with subsequent formation of polysulfide as intermediates. The rate of the reaction was found to decrease as HS ion in solution decreases and hence the optimal pH for oxidation was determined to be 7. They postulated the following mechanism for the second term in the rate expression ... 

The formation of higher polysulfides by the reaction of the intermediate HSOH continues in subsequent steps until HSg- is formed. The final step involves the formation of c-S8 (the only stable form of sulfur at STP) by an intermolecular displacement of HS9. Considering the complexity of the mechanism, the more appropriate overall stoichiometry is ... 

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