Sulfur intermediate oxidation states

The action of the Z-R reagent is first to supply an adequate concentration of Mn(II), which reacts with local excesses of permanganate and ensures the reduction of intermediate oxidation states of manganese to Mn(III). The Mn(II) also depresses the potential of the reversible Mn(III)-Mn(II) couple. Phosphoric acid (and to a lesser extent, sulfuric acid) also lowers the Mn(III)-Mn(II) potential, so that Mn(III) is reduced by Fe(II) rather than by chloride. Schleicher S stressed the importance of the Mn(III)-Mn(II) couple and maintained that five Mn(II) ions should be present locally for each Mn(VII) ion, to ensure that no manganese oxidation state higher than Mn(III) can exist. For this purpose four Mn(II) ions should suflBce ... 

In the process of reducing S04 or oxidizing sulfide, a number of compounds with intermediate oxidation state can be formed in addition to solid such as SO3 , 8203 , and 8405. The production of these intermediates is usually under biological control. Colloidal sulfur can form polysulfides such as HS, " or with HS. ... 

In aquatic settings, sulfate reducers are intimately associated with various heterotrophs and autotrophs. These include purple and green sulfur bacteria and thiobacUli which affect the availability of organic matter and alter the distribution of sulfur compounds in their various valence states. In the majority of the sites where elemental sulfur is formed (see Chapter 6.2), oxidation to sulfate also occurs and other intermediate oxidation states may also be found. For example, Volkov et al. (1972) reported the presence of thiosulfate in waters of some sediments from the Pacific Ocean east of Japan. These are interpreted as oxidation products of sulfide rather than intermediates of sulfate reduction since their concentration increased with increasing free sulfide ion content. 

Because sulfoxide sulfur is in an intermediate oxidation state, formally -1-4, it undergoes a small degree of thermal disproportionation to the corresponding sulfone and sulfide, at elevated temperatures near or above the normal boiling point. The formation of dimethyl sulfide is in part responsible for the garlic-like odor in aged DMSO, and can be minimized by storage under air. 

The pathways of sulfide oxidation in nature are varied, and in fact poorly known, but include (1) the inorganic oxidation of sulfide to sulfate, elemental sulfur, and other intermediate sulfur compounds, (2) the nonphototrophic, biologically-mediated oxidation of sulfide (and elemental sulfur), (3) the phototrophic oxidation of reduced sulfur compounds by a variety of different anoxygenic phototrophic bacteria, and (4) the disproportionation of sulfur compounds with intermediate oxidation states. The first three of these are true sulfide-oxidation pathways requiring either the introduction of an electron acceptor (e g. O2 and NO3 ), or, in the case of phototrophic pathways, the fixation of organic carbon from CO2 to balance the sulfide oxidation. The disproportionation of sulfur intermediate compounds requires no external electron donor or electron acceptor and balances the production of sulfate by the production of sulfide. This process will be taken up in detail in a later section. A cartoon depicting some of the possible steps in the oxidative sulfur cycle is shown in Figure 6. 

Fractionation also occurs during processes in the oxidative part of the sulfur cycle. Sulfide oxidation, which is pervasive in marine enviromnents (for example, 90% or more of the sulfide produced during sulfate reduction in coastal sediments are reoxidized) includes various processes, which are all associated with inverse isotope fractionation effects. These processes include the phototrophic oxidation by a variety of anoxygenic bacteria (-2 to 0%o), the non-phototrophic oxydation (up to -18%o), and the already mentioned disproportionation of sulfur compounds with intermediate oxidation states as... 

The history of sulfoxide photochemistry dates back at least to the early 1960s, but this important hmctional group has received substantially less attention than some of the other chromophores whose chemistry was explored in those years [1,2], On the other hand, the sulfoxide s chiral namre has brought its thermal chemistry into greater exposure [3-5]. Much of that is due to the relative ease of preparation of optically pure samples and their utility as chiral auxiliaries, directing the stereochemistry of subsequent synthetic steps. Also, the sulfoxide is an intermediate oxidation state of sulfur, which can be made achiral or to have different reactivity by easily achievable oxidations and reductions. 

The sulfur-rich oxides S 0 and S 02 belong to the group of so-called lower oxides of sulfur named after the low oxidation state of the sulfur atom(s) compared to the best known oxide SO2 in which the sulfur is in the oxidation state +4. Sulfur monoxide SO is also a member of this class but is not subject of this review. The blue-green material of composition S2O3 described in the older literature has long been shown to be a mixture of salts with the cations S4 and Ss and polysulfate anions rather than a sulfur oxide [1,2]. Reliable reviews on the complex chemistry of the lower sulfur oxides have been published before [1, 3-6]. The present review deals with those sulfur oxides which contain at least one sulfur-sulfur bond and not more than two oxygen atoms. These species are important intermediates in a number of redox reactions of elemental sulfur and other sulfur compounds. 

The dispiro compound A reacts with 2 cage molecules B to form the complex molecule 77 displayed in Fig. 13. The intermediate in brackets cannot be isolated. In contrast to the reaction of the same stannylene with sulfur (Eq. (26)) the dispiro compound A cannot be isolated seperately. The mechanism of reaction (34) may of course be more complicated. The cage molecule B is discussed in more detail in Section 6.5. It should be noted that in 77 six tin atoms of two different oxidation states are combined. 

Nitrite reductase and sulfite reductase are enzymes found in choroplasts and in prokaryotes that reduce nitrite to ammonia and sulfite to sulfide (Scott et al., 1978). Sulfite reductase also catalyzes reduction of nitrite at a lower rate. Both enzymes contain a siroheme prosthetic group linked to an iron-sulfur cluster. In siroheme, the porphyrinoid moiety is present in the more reduced chlorin form. Because NO lies between nitrite and ammonia in oxidation state, it is a potential intermediate. 

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