Peroxide bonds homolytic cleavage

Free-Radical Formation. Hydrogen peroxide can form free radicals by homolytic cleavage of either an O—H or the O—O bond. 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

In the case of cytochrome P450, it has been proposed that 0-0 bond cleavage precedes attack on substrate (22). It is generally believed that this cleavage is heterolytic, although homolytic cleavage of peroxide derivatives by cytochrome P450 has also been observed (23). 

Homolytic cleavage of most a bonds may be achieved if the compound is subjected to a sufficiently high temperature, typically about 200 °C. However, some weak bonds will undergo homolysis at temperatures little above room temperature. Bonds of peroxy and azo compounds fall in this category, and such compounds may be used to initiate a radical process. Di-tert-butyl peroxide, dibenzoyl peroxide... 

The ferric peroxide intermediate could undergo either heterolytic or homolytic cleavage to give either a ferryl or diferryl oxene. In the path shown in Fig. 14, the organic radical is generated by reaction with the Fe center after heterolytic 0—0 bond cleavage. For RNRB2 this is the... 

The proposed mechanism for the degradation involves SET to the peroxide resulting in homolytic cleavage of the 0—0 bond. An Ol-centred radical led to the formation of the enone, while an 02-centred radical afforded the diol. 

These peroxide bonds, as shown in Equations 3 and 4, are stable at room temperature, but when the temperature is raised, the bonds undergo homolytic cleavage to form peroxy radicals (Reactions 5 and 6). There-... 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

Since the aquohydroxo complex is the active species the decrease of the reaction rate at pH >13 has been explained with the decrease of the concentration of this reacting form. The first step of the reaction is the homolytic cleavage of Co-OH bond, followed by deprotonation of the resulting hydroxo radical with formation of O-", which eventually dimerizes to give hydrogen peroxide, as revealed by iodometry. Deprotonation of the hydroxo radical is necessary to make less effective the backward reaction of recombination with the Co2 + complex. 

Pathway 1 may not be relevant to Tyr catalysis homolytic cleavage of the peroxide O—O bond is presumably not the case in Tyr because the two copper ions are held by the protein chains so as to reverse the O—O bond cleavage. But pathway 2 can occur in Tyr as well. A proposal for the mechanism of Tyr is illustrated in Fig. 22. In Tyr, only one phenol can be inserted into the substrate-binding pocket in the protein,... 

Decomposition of 84 in benzene at 80° yields o-phenylbenzoic acid (50%), polymeric acid (25%), phthalic acid (7%), benzoic acid (5%), carbon dioxide (78%), and a little 3,4-benzocoumarin and diphenyl.72 The first step in this induced decomposition is homolytic cleavage of the peroxide bond.72... 

Free radicals may be generated by oxidation, reduction, or by homolytic cleavage of one or more covalent bonds, such as C—C bonds e.g. dimers of triarylmethyl radicals), N—N bonds e.g. tetrasubstituted hydrazines), O—O bonds e.g. hydroperoxides, dialkyl and diacyl peroxides, peroxycarboxylic esters), C—N bonds e.g. dialkyl azo compounds), and N—O bonds (as in the thermolysis of nitrogen pentoxide O2N— O—NO2). Two typical examples, which have been investigated in different solvents, are given in Eqs. (5-56) and (5-57) cf. also reaction (5-39a) in Section 5.3.2. 

Because the HO-OH bond in hydrogen peroxide is weak (AHdbe = SlkcalmoH vs. 90kcalmoH for the H-OOH bond), there has been a tendency to assnme homolytic cleavage in the reactivity and activation of HOOH. However, a pioneering study established that sulfur dioxide is converted to sulfuric acid through a nucleophilic addition mechanism (equation 79). 

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