Sulfate radical

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). 

The word alum is derived from the Latin alumen, which was appHed to several astringent substances, most of which contained aluminum sulfate (20). Unfortunately, the term alum is now used for several different materials. Papermakers alum or simply alum refers to commercial aluminum sulfate. Common alum or ordinary alum usually refers to potash alum which can be written in the form K SO Al2(SO 24H20, or it can refer to ammonium alum, ammonium aluminum sulfate. The term is also appHed to a whole series of crystallised double sulfates [M(l)M (lII)(SO 2 12H20] having the same crystal stmcture as the common alums, in which sodium and other univalent metals may replace the potassium or ammonium, and other metals may replace the alurninum. Even the sulfate radical may be replaced, by selenate, for example. Some examples of alums are cesium alum [7784-17-OJ,... 

Kolthoff et al. [42] showed that the thermal decomposition of peroxydisulfate ion yielded two free sulfate radicals according to the following reaction ... 

In some cases, due to the highly polar character of the sulfate radicals, peroxydisulfate initiators can provide slow polymerization rates with some apolar monomers since the polar sulfate radicals cannot easily penetrate into the swollen micelle structures containing apolar monomers. The use of mercaptans together with the peroxydisulfate type initiators is another method to obtain higher polymerization rates [43]. The mercaptyl radicals are more apolar relative to the free sulfate radicals and can easily interact with the apolar monomers to provide higher polymerization rates. 

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). 

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). 

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. 

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. 

When the initiation and termination reactions are the reverse of one another, the kinetic form is usually simpler than when the two are independent. Also, the transition-state composition follows directly from the rate law, which is why the term well-behaved is applied. Imagine, for example, that the termination step in the system most recently presented was the recombination of two sulfate radical ions rather than Eq. (8-38) ... 

Anipsitakis GP, DD Dionysiou (2003) Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ Sci Technol 37 4790-4897. 

Anipsitakis GP, DD Dionysiou, MA Gonzalez (2006) Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions. Environ Sci Technol 40 1000-1007. 

Beitz T, W Bechmann, R Mitzner (1998) Investigations of reactions of selected azaarenes with radicals in water. 1. Hydroxyl and sulfate radicals. J Phys Chem A 102 6760-6765. 

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.

Persulfate ions in aqueous solution when heated decompose to sulfate radical ions (1 0 which may react with water (15) or a hydroxyl-containing substrate (16) ... 

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. 

Carboxylate ions of saturated acids react with the sulfate radicals, giving rise to carboradicals (Eberson et al. 1968) ... 

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... 

---