Sulfur dioxide radical oxidation

Microsomes are capable of oxidizing not only organic substrates but also inorganic ones. An interesting example is the metabolism of bisulfite (aqueous sulfur dioxide) in microsomes. Although mitochondrial sulfite oxidase is responsible for the in vivo oxidation of bisulfite by a two-electron mechanism, cytochrome P-450 is also able to reduce bisulfite to the sulfur dioxide radical anion [56] ... 

As is the case with sulfur dioxide, nitric oxide undergoes a complex variety of reactions by which it is converted to nitrogen dioxide. For example, it too reacts with hydroperoxyl radicals to form nitrogen... 

In the first step, lattice sulfide is oxidized to the S radical (Eq. 22), which then reduces oxygen according to Eq. 23. The resulting sulfur dioxide radical anion is... 

Although some sulfuric acid is emitted directly by fuel-burning equipment, most of the sulfur in fuel is oxidized to and emitted as sulfur dioxide (S02). Sulfur dioxide contains sulfur in the ( + IV) oxidation state and dissolves in water to form sulfurous acid (H2S03), a relatively weak acid. In the presence of hydroxyl radicals in the atmosphere, sulfur dioxide is oxidized to sulfur trioxide (S03), which contains sulfur in the (+VI) oxidation state. Sulfur trioxide reacts with water to form H2S04, a strong acid. 

In the first step, lattice sulfide is oxidized to the S radical (Eq. 22), which then reduces oxygen according to Equation 23. The resulting sulfur dioxide radical anion is finally oxidized to the sulfate ion (Eqs. 24, 25). Note the surprising feature that oxygen is reduced via a preceding primary oxidation step (Eq. 22). The intermediate S radical has been detected at ZnS powder by ESR [93,94] and at colloidal zinc and cadmium sulfide by pulse radiolysis with UV-VIS detection (absorption maximum around 500 nm) [26, 95]. However, the sulfide radical anion can also be formed via the primary reduction S- -e S when elemental sulfur is present as impurity. 

Hydroxyl radicals, generated from hydrogen peroxide and titanium trichloride, add to the sulfur atom of 2-methylthiirane 1-oxide leading to the formation of propene and the radical anion of sulfur dioxide (Scheme 102) (75JCS(P2)308). 

Besides a parent ion, the mass spectra of benzo- and dibenzothiepins show the corresponding naphthalene or phenanthrene radical cations as the base peak.2-16 The mass spectra of 1-benzo-thiepin 1-oxides and 1,1-dioxides show the same naphthalene radical cation, formed by loss of sulfur monoxide or sulfur dioxide, respectively.14 In contrast, in the mass spectrum of 2,7-di-terf-butylthiepin peaks resulting from the loss of sulfur are not found.17... 

Two-component methods represent the most widely applied principles in sulfone syntheses, including C—S bond formation between carbon and RSOz species of nucleophilic, radical or electrophilic character as well as oxidations of thioethers or sulfoxides, and cheletropic reactions of sulfur dioxide. Three-component methods use sulfur dioxide as a binding link in order to connect two carbons by a radical or polar route, or use sulfur trioxide as an electrophilic condensation agent to combine two hydrocarbon moieties by a sulfonyl bridge with elimination of water. 

R23 is the only significant removal process for N02 and serves as well as a radical sink reaction for HO. Sulfur dioxide (with higher water solubility than NO2.) is also oxidized to sulfuric acid in aerosols and fog droplets (71,72,73,74) its gas-phase oxidation via R24 does not constitute a radical sink, since H02 is regenerated. 

Hydrogen sulfide in the air is oxidized at a relatively slow rate by molecular oxygen (02) but at a much faster rate by hydroxide (OH) radicals, forming the sulfhydryl radical and ultimately sulfur dioxide or sulfate compounds (Hill 1973 NSF 1976). Sulfur dioxide and sulfates are eventually removed from the atmosphere through absorption by plants and soils or through precipitation (Hill 1973). 

The resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

E. HamUton, personal communication), Calvert estimated sulfur dioxide oxidation rates by hydroperoxy, methylperoxy, hydroxyl, and methoxy radicals to be 0.85, 0.16, 0.23-1.4, and 0.48%/h, respectively. 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

The chief precursors for both oxidant and suspended particulate matter formation in the atmosphere, which are directly emitted into the atmosphere, are nitrogen oxides, hydrocarbons and their derivatives, ammonia, and sulfur dioxide. The measurement of particulate components is discussed in Chapter 2. This section describes briefly the measurement of nitrogen oxides, hydrocarbons, free radicals, and other precursors. 

In the upper atmosphere such oxidation of sulfur dioxide to its trioxide forming sulfuric acid or sulfate anion may occur at ambient temperature at a much slower rate in the presence of various free radicals. 

Margitan, J. J., Mechanism of the Atmospheric Oxidation of Sulfur Dioxide. Catalysis by Hydroxyl Radicals, J. Phys. Chem., 88, 3314-3318 (1984). 

The reaction of alkylene oxides or epoxides with sulfur dioxide to give cyclic sulfites is effected by carrying out the reaction at about 150°C for 4 hr at 2000 atm of S02 [28]. Pyridine is used in small amounts as a polymerization inhibitor. In addition, it has been reported that free radical-producing catalysts give improved yields and allow the reaction to be carried out at lower temperatures [28] (Eq. 19). 

In combustion, we have seen that fuel-sulfur is rapidly oxidized to sulfur oxides, primarily SO2. Sulfur dioxide may subsequently interact with the O-H radical pool in the postflame region. This interaction is important for two reasons. Sulfur dioxide is known to catalyze the recombination of the main chain carriers in the flame through the sequence... 

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