Sulfite, autoxidation

The autoxidation of aqueous solutions of sulfur dioxide (sulfite, bisulfite) is a classic problem in chemistry. Basic features of this reaction have been known since early in this century, when it was established that the reaction is trace metal ion catalyzed (1 ) and most likely involves free radicals (2). Certain chemical effects associated with sulfite autoxidation were noted also. Before the turn of the century, it was noted that sulfite would induce the oxidation of transition metal ions (3) and it was reported later that the oxidation of organic compounds was brought about during sulfite autoxidation ( 0. Conversly, organic compounds were also shown to serve as inhibitors of sulfite autoxidation (5). 

In sulfite autoxidation, an exceedingly important reaction is that of with S0n2 /HS0 . We have been able to derive a rate... 

Since both OH and SO, - very rapidly oxidize S032 to S03, these paths are indistinguishable in our experiments, although the extensive rearrangement required makes the final path highly unlikely. The identity of the correct path is quite important since the production of HSO " can lead to possible chain branching steps and the formation of S0 has consequences for the effect of inhibitors on sulfite autoxidation. 

It has been apparent for some time that the inhibition of sulfite autoxidation by organic compounds is due to their reactions with free radical intermediates. Further, it seems likely that the chemical effects associated with S02 autoxidation are due to these radicals or, possibly, peroxymonosulfate. Now that the reactivities of the likely free radical intermediates are known, the mechanism of these effects can be begun to be understood. For many organic compounds, like hydroquinone and other phenolic species, reaction with S03 and SO " is possible. Indeed, they prove to be the most efficient inhibitors of S02 autoxidation and the order of... 

The formation of free radicals can be used as a means of promoting sulfite autoxidation. For example, N02 reacts with many compounds by one-electron oxidation (16)... 

The branching ratio for this reaction has been a matter of some contention. The early and widely accepted free radical mechanism for sulfite autoxidation included only the termination step [112]. An investigation of the decomposition of HSOs" catalyzed by Ag and S20g ", however, led to calculated branching ratios, of 8.6 at 10°C, (7.3 and 8.8) at 17°C, and 9.7 at 25°C [113]. By... 

A brief overview on why most of the autoxidation reactions develop complicated kinetic patterns is given in Section II. A preliminary survey of the literature revealed that the majority of autoxidation studies were published on a small number of substrates such as L-ascor-bic acid, catechols, cysteine and sulfite ions. The results for each of these substrates will be discussed in a separate section. Results on other metal ion mediated autoxidation reactions are collected in Section VII. In recent years, non-linear kinetic features were discovered in some systems containing dioxygen. These reactions form the basis of a new exciting domain of autoxidation chemistry and will be covered in Section VIII. 

It should be stated here that the suggestion of Cu(III) as the oxidation state in different oxidation and autoxidation reactions does not exclude the participation of Cu(II)-Cu(I) couple in other reactions, where Cu(II) is easily reduced by the substrate-e.g., iodide or sulfite ions (4). Likewise, the Co(II)-Co(III) couple was shown to operate in the oxidation of cystein (69), which was shown to proceed according to... 

The autoxidation of S02 solutions is known to involve free radicals. Recent work on the reaction of free radicals with sulfite and bisulfite and on the reaction of the sulfite and peroxysulfite radicals is beginning to allow this complex system to be understood better. This is particularly true of the effect of added chemicals on S02 autoxidation and chemical transformations induced by this system. 

Recently, we have carried out studies on the free radical chemistry of sulfite. These studies have included kinetic measurements on the reactions of organic and inorganic free radicals with sulfite and bisulfite, and on the reactions of the sulfite derived radicals SO and S0 with organic and inorganic substrates. In this paper, I will review some of our results and results from other laboratories on the radical chemistry of sulfite and discuss these results in relation to the problem of S02 autoxidation. 

This result is important in modeling wet S0x-N0x flue-gas scrubbers. It shows that whereas N02 will oxygenate SO 2 " rapidly, it will not initiate the free-radical autoxidation of sulfite (1 7) Further, since the slightly soluble gas NO is the product, this reaction could reduce the efficiency of N0X removal (unless an NO scavenger is added). 

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... 

S04 ) and we have estimated its one-electron reduction potential to be about 1.1V at pH 7. Of particular interest is the ability of S05 to oxidize certain substances (aniline and dimethylaniline, for example) which form radicals which are capable of oxidizing sulfite. This can lead to the enhancement of the autoxidation of sulfite through the chain... 

In a mechanism for the iron catalyzed autoxidation of sulfite, this would be a chain branching step. We have some evidence that the reaction of Fe2+ with HSO, in acid, does lead to free radicals, but it is somewhat more complex than written above (24). 

The results we have obtained on the one-electron oxidation of sulfite and bisulfite by free radicals also are important in understanding possible chain-initiation and chain-carrying steps in S02 autoxidation. This is particularly true in systems which do not consist simply of S02 and a catalyst. For example, the production and subsequent reaction of halide free radicals could be important in any system containing halogen ions, and subsequent reactions of organic radicals with sulfite will be required to describe completely the effect of organic inhibitors. 

This could be important in some S0x-N0x scrubber systems (27). Further, some organic free radicals are formed easily by reaction with molecular oxygen. Reaction of these radcials with sulfite also can be used to initiate S02 autoxidation (12). 

Hayou, E., A. Treinin, and J. Wilf (1972). Electronic spectra, photochemistry and autoxidation mechanism of sulfite-bisulfite-pyrosulfite systems. J. Am. Chem. Soc. 94,47-57. 

Vinyl-substituted aromatic compounds such as a-methylstyrene,148 /S-isopropylstyrene,148 1-phenylcyclohexene,153 indene, 149 and 1,2-dihydronaphthalene151,152 give cyclic peroxides on autoxidation. For example, /3-isopropylstyrene gives the 1,2-dioxene derivative (149) together with polymeric peroxide. After the alkyl hydroperoxide formed by attack of oxygen on the isopropyl group had been reduced with sodium sulfite, 149 could be isolated in 26.4% yield by means of an alumina column. 

Franck and Haber introduced radically new ideas into the field of inorganic kinetics (11). They postulated the formation of OH radicals in the course of the ultraviolet irradiation of aqueous solutions of halides and sulfites and also in the autoxidation of sulfites. At the same time OH radicals were postulated by Urey,... 

Considering, for example, the autoxidation of benzaldehyde, the formation of one molecule of benzoic or of perbenzoic acid involves combination with one or two atoms of oxygen, respectively. In the autoxidation of an alkali sulfite, each molecule of sulfate formed utilizes one bound atom of oxygen. Thus,- from the analyses, the oxidation yield, RO, is obtained and is -reported as the ratio (multiplied by 100 in order to make the comparison easier) of the amount of reacted oxygen in excess to the amount of ozone consumed. 

Autoxidation of aqueous H2S solutions results in elemental sulfur, sulfite, thiosulfate, and eventually sulfate. However, under certain conditions monodispersed sulfur sols with particle diameters of between 0.2 and 1.2 /um are obtained [48]. For example, a solution of H2S of concentration 46 mmol 1 in pure H2O exposed to air at 25 °C became turbid after an induction period of ca. 8 h and then showed briUiant higher-order Tyndall spectra. A solution saturated in H2S exhibited a pH value of 4.1 and was rapidly oxidized in an atmosphere of pure O2. At pH values >8 no elemental sulfur was formed (clear colorless solution), in the range pH=7-8 yellow solutions were obtained (polysulfide anions), while at pH<7 turbid solutions were produced but the induction period was as longer as lower the pH value [48] ... 

Alternatives to Sulfites. To meet the needs of the food industry for alternatives to sulfites, a number of browning inhibitor formulations have been marketed. These products are mostly combinations of AA, EA, or their sodium salts with such adjuncts as citric acid, sodium or calcium chloride, phosphates, cysteine and potassium sorbate (23-29). Commercial browning inhibitor formulations vary widely both in AA or EA content and in recommended use levels (Sapers, G. M., Eastern Regional Research Center, Philadelphia, PA, unpublished data). These sulfite substitutes are considered to be less effective than sulfites because they do not penetrate as well into the cellular matrix (11). Furthermore, AA is easily oxidized by endogenous enzymes (18) or by autoxidation, and in the course of its intended role as a browning inhibitor, may fall into a concentration range where it exerts pro-oxidant effects (30). To enhance their effectiveness, the sulfite substitutes may be used in conjunction with modified atmosphere or vacuum packaging (29,31). 

Autoxidation of catecholamines ->5oi Autoxidation of sulfite Oxidation of hydroxylamine ... 

In the studies cited above, only gross reaction equations are given. However, with modem knowledge we can speculate about aqueous oxygen chemistry and photo-catalytic-enhanced redox processes (Chapter 5.3.5) that reactive oxidants such as OH, O3 and H2O2 will elementarily react with HS (in the autoxidation process all these species are slowly produced from O2), similar to the sulfite oxidation (Chapter 5.5.2.2). 

Other zeolitic materials with adsorbed porphyrins on external surface include the immobilization of Sn(TMPyP)Cl2 andSn(TPyP)Cl2onzeoliteYforuseasa solid photosensitizer material in the photodegradation of organic substrates. Zeolite supported Fe[T(n-MPy)P] (n =2, 3 or 4) was explored in heterogeneous catalytic autoxidations of sulfite. 

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