Organosulfur, reduced

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

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. 

Despite the high toxicity of H2S, not all organosulfur compounds are particularly toxic. Many of the compounds have strong, offensive odors that warn of their presence, which reduces their hazard. 

Secondary antioxidants or hydroperoxide decomposers (see Scheme 2.1) are typified by organosulfur species having reducing properties such as sulfides and thioethers. Tertiary phosphites also fall into this category (see Scheme 2.9). 

These nitrogen analogs of sulfoxides and sulfones are still relatively unusual types of organosulfur compound but a few methods are known for their reduction. Colonna and coworkers, for example, found that A -tosylsulfimides could be reduced to sulfides in good yields, using formamidinesulfinic acid (thiourea 5,5-dioxide) (6) under phase-transfer conditions at 70 °C (equation 16). This reagent was later... 

Lawson, L.D. (1993) Bioactive organosulfur compounds of garlic and garlic products Role in reducing blood lipids. In Human Medicinal Agents from Plants. American Chemical Society, Washington, DC, pp. 306-330. 

In conjunction with XANES and X-ray diffraction, initial investigations involving synchrotron based X-ray microspectrometry (at ESRF) has revealed two types of reduced sulfur compounds in the timbers of the Mary Rose. Organosulfur compounds are found in the lignin-rich middle lamella between wood cell walls, mostly as thiols, disulfides and elemental sulfur, while inorganic iron sulfides including iron pyrite, occurs in separate particles in the cell lumen. 

To prevent oxidation of reduced organosulfur in the wood cell walls, conservators are currently investigating antioxidants. Commercial antioxidants (ascorbic acid, anthocyanidins, proanthocyanins, flavon-3-ols, flavonones and flavonols) are being used to prevent oxidation of reduced species by mopping up free radicals. 

Sulfur Sulfur is present in carbonaceous materials as elemental sulfur, inorganic species, and organosulfur compounds. Its content varies between 0 and 5%. Carbon-sulfur complexes are extremely stable [121] and are not removed completely from the carbon matrix even at temperatures reaching 1373 K. Only heating in a reducing hydrogen atmosphere is able to remove sulfur completely from the carbon matrix [121]. 

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