Hydrogen sulfide acceptors

The process implications of equation 3 go beyond the weU-known properties (27—29) of NMP to faciUtate S Ar processes. The function of the aminocarboxylate is also to help solubilize the sulfur source anhydrous sodium sulfide and anhydrous sodium hydrogen sulfide are virtually insoluble in NMP (26). It also provides a necessary proton acceptor to convert thiophenol intermediates into more nucleophilic thiophenoxides. A block diagram for the Phillips low molecular weight linear PPS process is shown in Eigure 1. 

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

The genus Thiobacillus, especially the species T. denitrificans catalyzed the oxidation reactions of hydrogen sulfide yielding soluble hydrosulfide compounds, elemental sulfur, and sulfuric acid. Carbonyl sulfide and carbon disulfide are converted to hydrogen sulfide by hydrolysis. Additionally, they are oxidized to SOx and sulfates via microbial action. The reported oxidation reactions of thiosulfate using nitrate as electron acceptor are ... 

The sewer is dominated by heterotrophic microorganisms that degrade and transform wastewater components. These processes proceed under redox conditions determined by the availability of the electron acceptor. The importance of the processes for the sewer and the surroundings is not just caused by the removal and transformation of organic substrates — the electron donor—but is also a result of transformation of the electron acceptors exemplified by the formation of hydrogen sulfide from sulfate. 

While 02 serves as the electron acceptor in aerobic biodegradation processes forming H20 as the final product, degradation in anaerobic systems depends on alternative electron acceptors such as sulfate, nitrate or carbonate, which yield, ultimately, hydrogen sulfide (H2S), molecular nitrogen (N2) and/or ammonia (NH3) and methane (CH4), respectively. 

By far most of the reports on addition reactions of hetero-nucleophiles to activated dienes deal with sulfur-nucleophiles17,48,80,120-137, in particular in the synthesis of 7/3-sulfur-substituted steroids which, like their carbon-substituted counterparts (Section n.A), are of interest because of their ability to inhibit the biosynthesis of estrogens80,129-137. Early investigations17,120-122 concentrated on simple acyclic Michael acceptors like methyl sorbate and 2,4-pentadienenitrile. Bravo and coworkers120 observed the formation of a 3 1 mixture of the 1,6- and 1,4-adduct in the reaction of methyl sorbate with methanethiol in basic medium (equation 39). In contrast to this, 2,4-pentadienenitrile adds various thiols regioselectively at C-5, i.e. in a 1,6-fashion (equation 40)17,121,122, and the same is true for reactions of this substrate with hydrogen sulfide (equation 41), sodium bisulfite and ethyl thioglycolate17. 

Hydrogen sulfide is produced during anaerobic respiration (fermentation). Anaerobic respiration enables organisms, primarily bacteria and other microbes, to meet their energy needs using sulfate, elemental sulfur, and sulfur compounds as electron acceptors instead of oxygen. 

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

Then, electron acceptors are sequentially used in the following general descending order dissolved dioxygen, nitrate, sulfate, and carbon dioxide. Therefore, hydrogen sulfide formation follows nitrate reduction but precedes methane production as shown in Table 7.6. 

Sulfoxides. The complexes of bromine with tertiary amines have been used to convert sulfides to sulfoxides. The reaction can be conducted more conveniently with bromine and potassium hydrogen carbonate as the hydrogen bromide acceptor under two phase conditions (H2O-CH2CI2). Yields are in the range 80-100%. ... 

The reaction of carbon disulfide with 1,2-alkylene diamines (I) yields N-(2-aminoethyl) dithiocarbamic acids (II) which split off hydrogen sulfide thermally to give imidazolidine-2-thiones (III) (Hofmann-process). The simplest example, the reaction of carbon disulfide with ethylenediamine, is described in Organic Synthesis (5). The reaction is general for N,N -dialkyl-, N,N -diaryI, as well as for N,N -bis-(arylakyl) ethylene diamines. The rate of reaction is determined by the basicity of the diamine. Electron-donor substituents in the para-position of N-aromatically substituted ethylene diamines accelerate dithiocarbamate formation electron-acceptor groups retard it. 

Although molecular oxygen was most likely absent on the prebiotic earth, there was probably no dearth of potential electron acceptors in the mineral world at the time. Sulfate and its reduction product, sulfite, which serve as electron acceptors for some bacteria, are possible acceptors. This is an attractive possibility because sulfite is a common component of volcanic fumes, and its reduction could have yielded hydrogen sulfide. 

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