Bisulfide formation reactions

In some cases, the reaction affords a non-negligible amount of dipyridyl-disulfide and dialkyl dimers, particularly when the photolysis is run at the lower temperature of 80°C. One explanation can be the recombination of the thiopyridyl radicals and the stabilized (alkyl) radicals, respectively, suggesting that this process competes or even supplants the chain pathway at low temperature. Evidence was found, however, that bisulfide formation is rather the consequence of chain reactions of the type depicted in Scheme 5, in which the PyS reacts with its precursor and leads to further production of R. Arguably, this process is even more dominant where the steady-state concentration of the radicals are lower and the concentration of the starting ester is higher. Yet, increasing the temperature (to 110°C) is sufficient to simplify the reaction course to the normal radical chain process. ... 

In the reaction of reductive dissolution of metal oxides, the concentrations of the dissolved metals (Mn and Fe ) have been fixed at 1 [>,M to more accurately reflect their relative redox properties. This reaction is listed out of order so as not to separate it from the reactbn of formation of the bisulfide bn, HS. ... 

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 thiols appears related to the oxidaton of sulfide or sulfide minerals in Great Marsh, Delaware because of high sulfate excesses whenever thiols are present. Conversely, reaction of bisulfide ion with labile organic compounds such as... 

The absorption reactions are those of H2S and HCN combining with ammonia to form ammonium bisulfide and ammonium cyanide, followed by the oxidation of the ammonium bisulfide by the naphthoquinone (NQ) to form elemental sulfur and the naphthohydro-quinone (H2NQ). The reactions with oxygen take place in the regenerator and lead to the formation of NQ, thiocyanate, thiosulfate, and sulfate. Part of the elemental sulfur reacts with the available NH4CN in the oxidizer to produce NH4SCN. 

Note that this reduction reaction is the reverse of the photos)mthetic reaction shown in Eq. (2.51b). This reaction produces sulfide species, such as H2S (hydrogen sulfide), HS (bisulfide), and (sulfide ion), which are the most reduced forms of sulfur, with sulfur in the (—11) oxidation state. Sulfides are toxic most popularly known is H2S, which forms in rotten eggs and gives them their characteristic pungent odor. Sulfides are also important in causing the chemical precipitation of many metals, such as iron, copper, lead, and zinc, which form solids (such as FeS, CuS, PbS, and ZnS) upon reaction with. Precipitation with sulfide promotes long-term retention of metals in bottom sediments, and enables the geologic formation of many metal ore deposits. 

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