Sulfate radical, oxidant

Figure 28.21 The reactions of R u (11) pby 3 + are catalyzed by light at 452 nm that begins by forming an excited state intermediate. In the presence of persulfate, a sulfate radical is formed concomitant with the oxidative product Ru(III)bpy33+. This form of the chelate is able to catalyze the formation of a radical on a tyrosine phenolic ring that can react along with the sulfate radical either with a nucleophile, such as a cysteine thiol, or with another tyrosine side chain to form a covalent linkage. The result of this reaction cascade is to cause protein crosslinks to form when a sample containing these components is irradiated with light.

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

If the snlfate anion-radical is bonnd to the snrface of a catalyst (sulfated zirconia), it is capable of generating the cation-radicals of benzene and tolnene (Timoshok et al. 1996). Conversion of benzene on snlfated zirconia was narrowly stndied in a batch reactor under mild conditions (100°C, 30 min contact) (Farcasiu et al. 1996, Ghencin and Farcasin 1996a, 1996b). The proven mechanism consists of a one-electron transfer from benzene to the catalyst, with the formation of the benzene cation-radical and the sulfate radical on the catalytic snrface. This ion-radical pair combines to give a snrface combination of sulfite phenyl ester with rednced snlfated zirconia. The ester eventually gives rise to phenol (Scheme 1.45). Coking is not essential for the reaction shown in Scheme 1.45. Oxidation completely resumes the activity of the worked-out catalyst. 

This reaction resembles decarboxylation of carboxylates during electrode one-electron oxidation (Kolbe reaction). Kolbe reaction also consists of one-electron oxidation, decarboxylation, and culminates in dimerization of alkyl radicals just after their formation at the electrode surface. When the sulfate radical acts as a one-electron oxidant, the caboradical dimerization is hampered. The radicals can be used in preparative procedures. One typical example is alkylation of heterocyclic nitrogen bases (Minisci et al. 1983). This difference between Kolbe reaction and the reaction with the help of a dissolved electrode (the sulfate radical) deserves some explanation. The concentration of the one-electron oxidation products in the electrode vicinity is significantly higher than that in the bulk of the solution. Therefore, in the case of anode-impelled reactions, the dimerization of radicals produced from carboxylates proceeds easily. Noticeably, 864 secures the single electron nature of oxidation more strictly than an anode. In electrode reactions, radical intermediates can... 

Recently, the reaction between the sulfate radical anion and cyanuric acid, a nondegrad-able end product of the oxidative degradation of the triazine-based herbicide-atrazine, was reported. The degradation profile indicates that about 76% of the cyanuric acid has been decomposed after an absorbed y-radiation dose. It is therefore proposed that the reaction of peroxydisulfate could be utilized for the degradation of cyanuric acid in aqueous medium, which is important, since the latter is normally stable to further organic processes. 

At pH > 4, the oxidation is inhibited by organics, suggesting a free radical mechanism. One proposed mechanism, which originates in the work of Backstrom (1934), is shown in Table 8.7. The inhibition occurs when the organics react with the sulfate radical ion, S04". This inhibition has also been seen in laboratory experiments using fogwater collected in Diibendorf, Switzerland, where oxidation rates for S(IV) were less than expected based on the kinetics of the iron-catalyzed oxidation (Kotronarou and Sigg, 1993). 

We have investigated the reactions of the COs " radicals with double-stranded DNA by laser flash photolysis techniques [15]. In these time-re-solved experiments, the COs radicals were generated by one-electron oxidation of HCOs by sulfate radical anions, SO4 the latter were derived from the photodissociation of persulfate anions, S20s initiated by 308-nm XeCl excimer laser pulse excitation. In air-equilibrated buffer solution containing the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT), 208 , and an excess of HCO3., the decay of the CO3 radical anion absorption band at 600 nm is associated with the concomitant formation of the characteristic narrow absorption band of the G(-H) radicals near 310 nm. 

Experiments generating sulfate radicals, SO/", by UV photolysis of S2O82 in aqueous suspensions of silica nanoparticles showed a fast disappearance of the aqueous sulfate radicals yielding two transient species with absorption maxima around 320 and 600 nm, respectively [20]. The results indicated that at pH 3-9 S04 radicals build up an adduct on the surface with maximum absorption at 320 nm. This adduct shows similar reactivity to that observed for the sulfate radical in aqueous solution. The transients with absorption maximum at 600 nm were identified as SiO surface defects formed from the reaction between the adduct and deprotonated geminal and single silanols. Other less oxidative radicals lead to different radical-silica interactions. For example, thiocyanate radicals react with deprotonated silanols, not involving silanol oxidation. 

The reaction of hydrated electrons formed by radiolysis with peroxydisulfate yields the sulfate radical anion SO4 which is a strong chemical oxidant (Eqx = 2.4 V/NHE) [50, 58]. The oxidation of both purine and pyrimidine nucleotides by S04 occurs with rate constants near the diffusion-controlled limit (2.1-4.1 x 10 M s ). Candeias and Steenken [58a] employed absorption spectroscopy to investigate acid-base properties of the guanosine cation radical formed by this technique. The cation radical has a pKa of 3.9, and is rapidly deprotonated at neutral pH to yield the neutral G(-H) . Both G+ and G(-H) have broad featureless absorption spectra with extinction coefffcients <2000 at wavelengths longer than 350 nm. This has hampered the use of transient absorption spectra to study their formation and decay. Candeias and Steenken [58b] have also studied the oxidation of di(deoxy)nucleoside phosphates which contain guanine and one of the other three nucleobases by SO4 , and observe only the formation of G+ under acidic conditions and G(-H) under neutral conditions. 

In contrast, the decay of radicals in bimolecular reactions with other molecules (or ions), which are typically present at much higher concentrations than free radicals, does not depend on radical concentrations (or laser flash energy) and follows pseudo-first-order kinetics (4.16). An example of such a bimolecular reaction, is the one-electron oxidation (fc2S = 4.6 x 106M 1s 1) of bicarbonate anions by sulfate radicals [33] ... 

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. 

Among other Lewis acids, sulfur trioxide in dimethyl sulfate solution oxidizes perylene to the cation radical. Naphthacene is so easily oxidized that the dication is formed (Aalbersberg et al., 1959b). 

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