Formation of free radicals

Most of free radical polymerizations are started by free radicals that have been produced by the thermal decomposition of suitable free-radical-forming agents. Compounds whose bonds are easily broken make suitable free-radical-forming agents for example, radicals are formed by the decomposition of azo compounds such as azobisisobutyronitrile [reaction (20-3)] or by the degradation of compounds such as peroxides, peresters, peracids or hydroperoxides. Benzoyl peroxide can decompose into benzoyl-oxy radicals, and also, in certain solvents, into phenyl radicals  

In the presence of monomer, polymerization is generally started by the benzoyloxy and not the phenyl radicals. Potassium persulfate, K2S2O8, decomposes thermally into two radical anions, SO4, which initiate polymerization. 

The rate of decomposition of free-radical-forming agents can vary considerably (Table 20-1). Decomposition is facilitated when there are additional resonance stabilization possibilities for the free radical. However, the greater the resonance stabilization, the more stable will be the free radical. For example, pentaphenylcyclopentadienyl exists in the solid state in the form of up to 100% dissociated free radicals. 

The rate of decomposition also depends on the solvent. Benzoyl peroxide, for example, after 60 min at 79.8 C, is 13% decomposed in CCI4, 15.5% in benzene, 51% in cyclohexane, and 82.2% in dioxane. After only 10 min in i-propanol the decomposition is 95.1 %, and in amines decomposition occurs explosively. Thus, solvents can intervene in the free-radical-forming process. 

Alkyl radicals have particularly low resonance stabilizations (see also Section 20.2.1) and therefore induce polymerization very readily. However, they are so reactive that they do not confine themselves to reacting selectively with C=C double bonds (transfer reactions, see Section 20.3). Alkyl radicals can be produced, for example, by reacting tributyl boron with oxygen  

Since most organic molecules have all their electrons paired, conversion into radicals involves homolysis of a covalent bond, or a single electron transfer to or from a molecule. 

For molecules with a weak covalent bond, heating provides enough thermal energy to break the covalent bond homolytieally (one electron 

The energy required to break a covalent bond can also be provided by electromagnetic radiation, normally ultraviolet or visible light. 

Some reactions occur thermally at temperatures too low for homolysis of any of the covalent bonds present to provide enough radicals to start the reaction. For example, mixtures of fluorine and methane explode at room temperature, and many hydrocarbons are oxidized slowly by molecular oxygen (see below). It has been postulated that the bimolecular reactions (6.10) and (6.11) are responsible. In (6.10), the formation of the very strong H-F bond makes this reaction much less endothermic than the simple homolysis of the fluorine molecule. In (6.11), a strong O-H bond is formed in the hydroperoxyl radical 18, whereas two relatively weak bonds are broken, the 0=0 n bond and the C-H bond in 16 which is weakened by the stabilization of the product benzylic radical 17. The occurrence of these molecule-induced homolysis reactions is difficult to prove because the compounds formed tend to be swamped by those from the subsequent radical reactions. 

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Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Fenton chemistry is dependent on the formation of free radicals. 

The instabihty of tert-huty areneperoxysulfonates is increased by the presence of electron-withdrawing substituents on the aromatic ring and decreased by electron-donating substituents. However, even the most stable members decompose violently on warming, as indicated in Table 14. These peroxyesters appear to decompose heterolyticaHy without the formation of free radicals (44). 

Radiation Dosimetry. Radioactive materials cause damage to tissue by the deposition of energy via their radioactive emissions. Thus, when they are internally deposited, all emissions are important. When external, only those emissions that are capable of penetrating the outer layer of skin pose an exposure threat. The biological effects of radiation exposure and dose are generally credited to the formation of free radicals in tissue as a result of the ionization produced (17). 

There is a great deal of flexibility in the choice of laser radiation in the production of thin Aims by photochemical decomposition, and many routes for achieving the same objective can be explored. In most reactions of indusuial interest the reaction path is via tire formation of free radicals as intermediates, and the complete details of the reaction patlrs are not adequately defined. However, it may be anticipated that the success of the photochemical production of new materials in tlrin fllms and in fine powder form will lead to considerably greater effort in the elucidation of these kinetics. 

In the case of NO2, for each photon absorbed below 400 nm, photodissodation occurs. For other photoabsorbers, HNOj and aldehydes, the photodissociation process leads to the formation of free radicals. 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

The other pattern of breaking the carbon-carbon bonds which results in the formation of free radicals is observed to much lesser degree and is responsible for an insignificant propylene content upon thermal destruction. 

From ESR studies the formation of free radicals on the a-C atom of the amines attached to hydroxyalky group as ... 

Mn(III) is able to oxidize many organic substrates via the free radical mechanism [32], The free radical species, generated during oxidation smoothly initiate vinyl polymerization [33-35]. Mn(III) interacts also with polymeric substrates to form effective systems leading to the formation of free radicals. These radicals are able to initiate vinyl polymerization and, consequently, grafting in the presence of vinyl monomers. 

On the other hand, the decrement in grafting observed with oxalic acid is higher than that observed with sulfuric acid. This can be due to the fact that oxalic acid is more active than sulfuric acid for the formation of free radicals and, consequently, the grafting in the presence of oxalic acid is greater than in the presence of sulfuric acid. 

By further increasing the methanol in the grafting medium, the graft yield decreases. This can be related to the lower solubility of the initiator in the grafting medium and a reduced formation of free radicals, which... 

The initial step in the chemistry of thermal cracking is the formation of free radicals. They are formed upon splitting the C-C bond. A tree radical is an uncharged molecule with an unpaired electron. The rupturing produces two uncharged species that share a pair of electrons. Equation 4-1 shows formation of a free radical when a paraffin molecule is thermally cracked. 

A major complication in applying radiation chemical techniques to ion-molecule reaction studies is the formation of nonionic initial species by high energy radiation. Another difficulty arises from the neutralization of ions, which may also result in the formation of free radicals and stable products. The chemical effects arising from the formation of ions and their reactions with molecules are therefore superimposed on those of the neutral species resulting from excitation and neutralization. To derive information of ion-molecule reactions, it is necessary to identify unequivocally products typical of such reactions. Progress beyond a speculative rationalization of results is possible only when concrete evidence that ionic species participate in the mechanism of product formation can be presented. This evidence is the first subject of this discussion. 

For a summary of methods of radical formation, see Giese, B. Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds Pergamon Elmsford, NY, 1986, p. 267. For a review on formation of free radicals by thermal cleavage, see Brown, R.F.C. Pyrolytic Methods in Organic Chemistry Academic Press NY, 1980, p. 44. 

A free-radical process consists of at least two steps. The first step involves the formation of free radicals, usually by homolytic cleavage of bond, that is, a cleavage in which each fragment retains one electron ... 

It should be pointed out however that there is no proof for the intermediate formation of free radicals and other mechanisms may explain the product distribution equally well. Such an alternative may be an ionic mechanism, possibly catalyzed by impurities or the walls of the container. In the above experiments Pyrex tubes were used [64]. 

The catalytic hydrogenation of alkyl halides (RX) probably also proceeds via the intermediate formation of free radicals, which are formed in this case by the abstraction of a halogen atom (see Section III,B). 

One type of fatty liver that has been smdied extensively in rats is due to a deficiency of choline, which has therefore been called a lipotropic factor. The antibiotic puromycin, ethionine (a-amino-y-mercaptobu-tyric acid), carbon tetrachloride, chloroform, phosphorus, lead, and arsenic all cause fatty liver and a marked reduction in concentration of VLDL in rats. Choline will not protect the organism against these agents but appears to aid in recovery. The action of carbon tetrachloride probably involves formation of free radicals... 

Figure 15.11 Possible scheme for the formation of free radicals from the metabolism of dopamine. Normally hydrogen peroxide formed from the deamination of DA is detoxified to H2O along with the production of oxidised glutathione (GSSG) from its reduced form (GSH), by glutathione peroxidase. This reaction is restricted in the brain, however, because of low levels of the peroxidase. By contrast the formation of the reactive OH-radical (toxification) is enhanced in the substantia nigra because of its high levels of active iron and the low concentration of transferin to bind it. This potential toxic process could be enhanced by extra DA formed from levodopa in the therapy of PD (see Olanow 1993 and Olanow et al. 1998)...

Metals of transient valency, particularly copper and iron, catalyse the lipid oxidation because they decompose lipid hydroperoxides with formation of free radicals [15.8] and [15.9] ... 

The mechanism of carbon dioxide reduction in aqueous and nonaqueous solutions was investigated by several authors. It is now generally accepted that the reduction of carbon dioxide to formate ions is a multistep reaction with the intermediate formation of free radicals CO2 and HCO2 either in the solution or adsorbed on the electrode ... 

Turner, J.J.O., Rice-Evans, C., Davies, M.J. and Newman, E.S.R. (1991). The formation of free radicals by cardiac myocytes under oxidative stress and the effects of electron-donating drugs. Biochem. J. 277, 833-837. 

Once UV photons have been absorbed by the polymer, excited states are formed they disappear by various routes, one of them leading to the formation of free radicals by cleavage of the C-Cl bonds. The very reactive Cl radicals evolved are most likely to abstract an hydrogen atom from the surrounding CHC1 sites to generate a-B,B ... 

Catalase is a heme protein belonging to the class of oxidoreductases with ferripro-toporphyrin-IX at the redox center, and it catalyzes the disproportionation of hydrogen peroxide into oxygen and water without the formation of free radicals. 

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