Sulfate radical anions

The sulfate radical anion is formed by thermal, photochemical or redox decomposition of persulfate salts (41, sec 3.3.2.6.1). Consequently, it is usually used in aqueous solution. However, crown ether complexes or alkylammonium salts may be used to generate the sulfate radical anion in organic solution (see 3.3.2.6.1). 

In the case of electron-deficient monomers (e.g, acrylics) it is accepted that reaction occurs by initial addition of the sulfate radical anion to the monomer. Reactions of sulfate radical anion with acrylic acid derivatives have been shown to give rise to the sulfate adduct under neutral or basic conditions but under acidic conditions give the radical cation probahly by an addition-elimination process. 

Hydroxy radical and sulfate radical anion, though they may sometimes give rise to similar products, show quite different selectivity in their reactions with unsaturated substrates. In particular, the sulfate radical anion has a somewhat lower propensity for hydrogen abstraction than the hydroxyl radical. For example, the sulfate radical anion shows little tendency to abstract hydrogen from mcthacrylic acid.232 

Sulfate radical anion may be converted to the hydroxyl radical in aqueous solution. Evidence for this pathway under polymerization conditions is the formation of a proportion of hydroxy end groups in some polymerizations. However, the hydrolysis of sulfate radical anion at neutral pi I is slow (k— 107 M 1 s 1) compared with the rale of reaction with most monomers (Ar=l08-109 M 1 s 1, Table 3.7)440 under typical reaction conditions. Thus, hydrolysis should only be competitive with addition when the monomer concentration is very low. The formation of hydroxy end groups in polymerizations initiated by sulfate radical anion can also be accounted for by the hydration of an intermediate radical cation or by the hydrolysis of an initially formed sulfate adduct either during the polymerization or subsequently. 

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M 1 s 1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454 

The preferred initiation pathway i.s dependent on the particular monomer involved and the reaction conditions. Generally radical cation formation (by either mechanism) is facilitated by low pH. The failure to detect an intermediate sulfate adduct led workers to propose that reactions of the sulfate radical anion with electron-rich alkenes and S derivatives proceeded by pathway (B) over a wide range of pH and reaction conditions. However, other w orkers rationalized similar data by allowing the initial formation of a sulfate adduct (pathway A). Detection of an intennediate in the reaction of sulfate radical anion with S or 

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Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

It was found that the sulfate radical anion S04 produced photochemically in Scheme (46) is responsible for generating the cellulose derivative macroradicals by hydrogen abstraction, which added the vinyl monomer to produce the grafted copolymer. The main disadvantage of this method is the production of large quantities of undesirable homopolymers in addition to the grafted copolymers. 

Photolysis or thermolysis of persulfate ion (41) (also called peroxydisulfate) results in hoinolysis of the 0-0 bond and formation of two sulfate radical anions. The thermal reaction in aqueous media has been widely studied."51 232 The rate of decomposition is a complex function of pH, ionic strength, and concentration. Initiator efficiencies for persulfate in emulsion polymerization are low (0.1-0.3) and depend upon reaction conditions (Le. temperature, initiator concentration)."33... 

Persulfate (41) absorbs only weakly in the UV (e ca 25 M 1 cm 1 at 250 nm).242 Nonetheless, direct photolysis of persulfate ion has been used as a means of generating sulfate radical anion in laboratory studies.242 243... 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

Two pathways for the reaction of sulfate radical anion with monomers have been described (Scheme 3.81).252 These are (A) direct addition to the double bond or (B) electron transfer to generate a radical cation. The radical cation may also be formed by an addition-elimination sequence. It has been postulated that the radical cation can propagate by either cationic or a radical mechanism (both mechanisms may occur simultaneously). However, in aqueous media the cation is likely to hydrate rapidly to give a hydroxyelhyl chain end. 

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. 

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. 

Niehaus H, Hildenbrand K (2000) Continuous-flow and spin-trapping EPR studies on the reactions of cytidine induced by the sulfate radical-anion in aqueous solution. Evidence for an intermediate radical cation. J Chem Soc Perkin Trans 2 947-952 Niles JC, Burney S, Singh SP, Wishnok JS, Tannenbaum SR (1999) Peroxynitrie reaction products of 3, 5 -di-0-acetyl-8-oxo-7,8-dihydro-2 -deoxyguanosine. Proc Natl Acad Sci USA 96 11729-11734... 

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. 

Hydroxyl radical, phosphate radical anion, and sulfate radical anion add to the 5 position of oxazole <90JPC1887>. The hydroxyl radical adduct is stable at pH 5-9, but reacts further above or below that range to give a ring-opened product (Scheme 22). 

One surfactant is sodium lauryl sulfate, and the initiator is usually (NH4)2S20,. This forms the sulfate radical anion, which starts the process. Table E44-1, p. 663, shows a typical charge. 

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