Radicals sulfite

The (8O3) radical is essentially the sulfur trioxide anion-radical. It is named the sulfite anion-radical because it is obtained from sodimn sulfite on reaction with titanimn trichloride in water. This reaction usually proceeds in the presence of ethylene diamine tetraacetic acid as a complexing agent and hydrogen peroxide as an oxidant (Bradic and Wilkins 1984). Under these conditions, the 

To generate 8O3 in acidic mediums, the reaction between sodium hydrogen sulfite and cerium ammonium nitrate should be employed. In (NH4)2Ce(N03)g, cerinm has 4+ oxidation state. In acidic mediums, (NH4)2Ce +(N03)g easily oxidates the sulfite ion + 803 — + 

This method of 8O3 generation is nsed to perform the addition of the snlfite anion-radical to unsaturated compounds RCH=CHR + 8O3 — RCH(803 )CH R. 

Depending on the structnre of ethylenic componnds, addition of 8O3 to C=C bonds can be performed both in alkaline and in acidic solution. In acidic mediums, the addition proceeds easier than in the alkaline ones (Ozawa and Kwan 1985). 

The reactions of 8O3 anion-radicals with organic and inorganic compounds have commanded considerable interest because of the role of these anion-radicals in grievous health and technological 

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The carbon-oxygen double bond of the carbonyl group is opened, and the hydrogen sulfite radical is added. An increase in temperature reverses the reaction more easily for ketones than for aldehydes. 

Waygood, S. J., and W. J. McElroy, Spectroscopy and Decay Kinetics of the Sulfite Radical Anion in Aqueous Solution, J. Chem. Soc. Faraday Trans., 88, 1525-1530 (1992). 

Rate constants for the reactions of S0o with a wide variety of organic compounds are summarized in Table 2. The sulfite radical was found to oxidize ascorbate, trolox (a water soluble tocopherol derivative), methoxyphenol, hydroquinone and other phenolic compounds, sulfonated hydroquinones, phenylenediamines, and phenothiazines with rate constants ranging to 1 CrM 1 s. ... 

Possibly the most important reaction of the sulfite radical in autoxidation systems is with molecular oxygen. The reaction has been suggested to lead either to 02 or to the peroxy radical SO5... 

The formation of sulfite radical anion (i.e., S03 ) as an intermediate is indicated by the fact that S20 (2S03 - S20 ) is one of the end products when the illuminations are carried out under N2. 

The rate constant for the reaction of HS2 with 02 (R15) is expected to be higher than the rate constant for the reaction of HS2 with S032- (R14) a value of 5 X 107 M"1 s"1 was therefore used for fcrl5. Sulfate radical is a very strong oxidant and is expected to react with S(-II) near the diffusion-controlled limit the value of kr39 was set at 109 M"1 s"1. Sulfur dioxide and sulfite radicals are expected to react with S(-II) slower than S04 -, and a value of 108 M"1 s"1 was used for kr4l and kr42. Reaction R41 should be seen as multistep where the nucleophilic adduct SC HS2" formed by the reaction of S02 - and HS" reacts further with 02. 

S(IV) oxidation. The existence of the sulfite radical and its role in biological damage (whereby the subsequently produced peroxosulfate radical SOJ is a much stronger oxidant) has long been known (Neta and Huie 1985). Many molecules, radicals and metal ions react with sulfite and bisulfite in a one-electron oxidation (Table 5.21) ... 

The sulfite radical reacts quickly with oxygen to form the peroxosulfate radical ks.29s = 2.7 10 L mol s (Buxton et al. 1996) ... 

In aqueous solution, peroxo radicals can also react with bisulfite, gaining the sulfite radical ... 

A wide range of sulfur radicals have been reported. Sulfur dioxide (S02 ), sulfite radical (SO ), sulfate radical (SOJ ) and peroxomonosulfate radical (SOs") can all be formed from sulfur dioxide, which is an environmental air pollutant, as well as from sulfites and bisulfites used as preservatives [114-118] and methods of their production and their reactivity has been previously reviewed [119]. Briefly, SOa acts as a one electron reductant whereas the others are all oxidising species with being the strongest one electron oxidant [117-119]. 

The generation of SO/ involves a variety of reactions and the specific method can influence the ultimate ion chemistry. Dissociative electron attachment to SO2, which exhibits an energy-dependent cross-section (SO2 + e" —> SO" + O) [8], is the typical preparation method for SO". The sulfur dioxide radical anion, S02 , is easily produced via three-body electron attachment following ionization of dilute mixtures of SO2 in a chemically inert buffer gas [9]. Both O/ and CO," have been used [9J as efficient O" donors to form the sulfite radical anion, SO/, from sulfur dioxide, reactions (1) and (2) ... 

The sulfite radical anion, S03, is an important intermediate in the autoxidation of dissolved SO2, where it is formed in the one-electron oxidation of sulfite or bisulfite, for example, reaction (21) ... 

The two paths appear to be of approximately equal importance [45]. The photolysis of thiosulfate also has been demonstrated to generate the sulfite radical [46],... 

The sulfite radical is a mild one-electron oxidant. The reduction potential of this radical was first demonstrated to be less than 0.89 V [60] based on kinetic considerations. In a pulse radiolysis study, a transient equilibrium was established at pH 3.6 involving the sulfite radical and chlorpromazine, reaction (28) ... 

The sulfite radical is a sulfur-centered radical and would form a S-H bond if it undergoes a hydrogen abstraction reaction. These bonds are weaker than typical C-H bonds and indeed hydrogen abstraction reactions, for example from 2-propanol, have been found to be very slow k < 10 L mol" s ) [53]. SOf does, however, add to unsaturated bonds, not only carbon-carbon double and triple bonds, but also C=S and C=N bonds [68]. The rate constants for the addition of 803" to unsaturated fatty acids were found to increase from about 3x10 L mol" s" for non-conjugated, straight chain fatty acids (linoleate, linolenate, and arachidonate) to about 1 x lO L mol" s for branched chain, conjugated fatty acids (crocin and crocetin) [69]. 

The peroxysulfate radical, SO," is a key intermediate in the autoxidation of sulfite/bisulfite solutions, but is also the intermediate about which the least is known. It is formed subsequent to the one-electron oxidation of sulfite/bisulfite by the reaction of the sulfite radical with molecular oxygen (Reaction (29)). It is also generated in the metal ion-induced free radical decomposition of peroxy-monosulfate, H8O5" [107]. Its production in the oxidation of H8O5" by Ce(IV) has been confirmed by ESR spectroscopy [108], where a g factor of 2.0145 was... 

The peroxymonosulfate radical is a stronger one-electron oxidant than the sulfite radical and has been shown to react with a number of reductants more rapidly than does 803" (Table 3). Indeed this radical can oxidize some substances (for example, aniline and dimethylaniline) leading to radicals which can, in turn, oxidize sulfite ions. In these cases, the reduction potential of the substrate is intermediate between those of 803" and 805" (for aniline, = 1.03 V and for dimethylaniline, =0.86 V [109]). 8ome early kinetic results suggested that the one-electron reduction potential for 8O5 was about 1.1 V [ 110], while a calculation based on an estimated homolysis energy led to an estimate of 1.7 V... 

Both aliphatic [5, 44] and aromatic sulfonyls [28] have been shown to undergo this reaction in glasses at temperatures at which dissolved oxygen becomes mobile. At least two oxysulfonyl species were shown to enter this reaction as well. Those are the sulfite radical SO3" in solutions [84, 85] and glasses [28] and = SiOS02 [47] species on silica surface at room temperature. The addition was found to be reversible in the latter case. One more reaction with the rate constant 4.37 X 10" cm molecule" s" is reported to be involved in a gas phase oxidation of SO2 [86] ... 

Photoionization. - 2.1.1 Sulfite anion. The photoionization of the sulfite anion SOf was studied by Fessenden et a few years ago. The steady-state EPR spectra of the hydrated electron and the sulfite radical-anion SOs show no CIDEP effects and indicate the lifetime of the hydrated electron to be about 100 ps. The sulfite radical-anion is often used as g factor standard in photolysis EPR experiments [ [(SOs") = 2.00316]. Bussandri et al. studied the laser photolysis of sulfite ions in basic solution by FT EPR with very high time resolution. They observed the FT EPR spectra of hydrated electrons and sulfite radical-anions with absorption/emission (A/E) pattern caused by the radical pair mechanism (RPM CIDEP) with the electron line in emission [fif(eaq ) = 2.00044 at room temperature] and the sulfite radical-anion in absorption. In the time profiles of both lines, oscillations of the EPR intensities were observed in the first 300 ns. This coherent oscillation in both radicals is the first direct EPR observation of zero quantum coherence in freely diffusing radicals. Previously zero... 

The Marcus equation correlates these two reactions and earlier data for [Fe(CN)6] as well. The sulfite radical anion, SOJ, is proposed as a later intermediate, although this was not tested directly. Oxidations of SOs by other metal complexes believed to proceed via SOJ, follow second-order kinetics > at 25°C, fc = 5.6 x 10 ([IrClef ), 3.18 x lO ([IrBr< f ), and 2.1 X 10 ([Fe(bipy)3] ) M" s . ... 

Irradiation of 4-chloro-l-hydroxynaphthalene in aqueous solution of sodium sulfite caused the substitution of the chlorine atom by sulfite. The mechanism was rationalized as involving the sulfite radical anion in a chain S l process [207]. The reaction led to sulfonic acids in the photolysis of various para substituted anilines a halogen or a SOjX (X = CHj, NH, CFj) group was replaced, thus leading to the corresponding sulfonic acid [208]. As for the p-sulfonyl precursors, experiments with labelled sulfite confirmed that the formation of the products involved cleavage of the aryl-S bond. 

Ru(NH3)4(phen)] and [Ru(NH3)4(phen-4-S03)] . The mechanism for formation of the latter, involving both electron transfer and substitution at the coordinated phen ligand, is unclear [Ru(NH3)4(phen)] " does not react directly with the sulfite radical anion. Substitution at ruthenium(II) plays a key role in the catalytic aerobic oxidation of cyclohexene by 2,2 -bipyridyla-quaphosphine complexes such as the [Ru(OH2)(bipy)2(PR3)] cations/ ... 

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