Radicals formed by homolytic cleavage

Radicals form by homolytic cleavage of weak bonds... 

Dialkyl peroxides (dimethyl peroxide is shown in the table) contain the very weak 0-0 bond. The radicals formed by homolytic cleavage of these bonds, stimulated by a little heat or light, initiate what we call a radical chain reaction , which results in the formation of the Br radicals, which add to the alkene s C=C double bond. We shall return to radical chain reactions and their mechanisms in detail later in this chapter. 

Cation radicals of weak acids may react either by heterolytic cleavage (loss of a proton to produce a radical) or by homolytic cleavage (loss of a hydrogen to form a carboca-tion). Alkane cation radicals in liquid hydrocarbon solution undergo ion-molecule reactions, such as proton transfer on a submillisecond scale (Werst et al. 1990). 

Free radicals, formed by homolytic bond cleavage, may couple to groups on a receptor in several ways (Fig. 2.4A). Diradicals (e.g. triplet excited states) probably abstract an atom from the receptor (e.g. H ) and the two radicals that are formed couple (Fig. 2.4b). 

A radical formed upon homolytic cleavage of a functional initiator carries the corresponding function. Since upon initiation the primary radical adds to a monomer, this functional group remains attached to the formed polymer molecule. However, this functionalization procedure is far from being adequate for the following reasons — Functionalization at only one chain end requires that termination occurs exclusively by disproportionation (and not by recombination) and that transfer reaction of the radical to monomer and to solvent molecules can be disregarded. If the first condition is not fulfilled, functionalization at both chain ends can result. If the second condition is not valid, only a fraction of the formed macromolecules is functionalized, namely those arising from primary radicals. 

Free radicals can be formed by thermolytic cleavage, photolysis (nltravi-olet light photolysis of hydrogen peroxide to form hydroxyl radicals), radiolysis (ionizing radiation of water to form hydroxyl radicals), or by homolytic cleavage with the participation of another molecule (i.e., Fenton reaction). Perkins, J., Radical Chemistry The Eundamentals, Oxford University Press, Oxford, UK, 2000. 

Hexanal can also be obtained from 9-HPODE by cleavage of the peroxyl bond. The radical intermediate product is stabilized by fission of the 9,10-carbon bond to a radical and 2,4-decadienal. The radical formed by homolytic fission of the peroxyl bond in 9-HPOPDE (see Scheme 12) abstracts hydrogen from another molecule to generate octanoic acid. 2,4-decadienal suffers further degradation by adding water. The thus formed new intermediate is cleaved to hexanal and 2-butenal [206,207] (Scheme 12). 

A problem with hydroperoxides is that they can also form peroxidic radicals (RO) by homolytic cleavage of the peroxidic 0-0 bond or other pathways as advocated by the groups of Mansuy and Bruice [550,551]. This can lead to unwanted side reactions, including oxidative degradation of the porphyrin. 

The hydroxyl radicals may be formed by homolytic cleavage of hydrogen peroxide (Equation 12.1) or by metal ion catalyzed decomposition (Equations 12.2 and 12.3)... 

The photochemistry of ketyl radicals is currently of interest and has been reviewed.Photolysis of benzophenone ketyl radical in acetonitrile solution has been shown to yield a mixture of 0-protonated benzophenone, formed by electron ejection, and benzophenone, formed by homolytic cleavage of a hydrogen atom. In non-polar solvents the cyclic analogues of benzophenone ketyl radical such as (353) are also reported to undergo hydrogen atom loss and regeneration of the parent ketone upon excitation.Photolysis of triarylmethyl radicals has been described and leads to the production... 

Radicals are formed by homolytic cleavage or by reaction of a radical initiator with a paired-electron molecule. 

The ESI product ion spectrum of florfenicol (Scheme 4,1) does not show a molecular ion peak at m/z 358 (Fig. 10.4). The significant peak corresponding to the loss of water is formed by heterolytic fragmentation as shown in Scheme 4 (II, m/z 340). The II decomposes to III (m/z 320) by a neutral loss of HF, and this fragmentation is promoted by the formation of a substituted tropylium ion (Scheme 4). The product ion spectrum of florfenicol is characterized by the unusual feature of a most abundant peak occurring at odd mass, namely at m/z 241. The IV, a radical ion, is formed by homolytic cleavage of the sulfur—carbon bond in III, and loss of the methanesulfinic radical (Scheme 4). 

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

Figure 13.18 S-adenosyl methionine (SAM), a source of 5 -deoxyadenosyl radicals. SAM binds to the subsite iron (in blue) of the reduced [4Fe-4S] cluster via its a-aminocarboxylate group. The 5 -deoxyadenosine radical is formed by electron transfer which occurs either (a) by outer-sphere mechanism or (b) by p-sulfide alkylation followed by homolytic cleavage of the 5 -S-CH2Ado bond. In both cases, methionine is released. (From Fontecave et al., 2004. Copyright 2004, with permission from Elsevier.)...

The gaseous dichlorocarbene radical cation reacted with alkyl halides via a fast electrophilic addition to form a covalently bonded intermediate (CI2C—X—R)+ in a Fourier transform ion cyclotron resonance mass spectrometer. This intermediate fragments either homolytically or heterolytically to produce net halogen atom or halogen ion transfer product. Addition of carbonyls to the carbene ion is followed by homolytic cleavage of the C-O bond to yield a new carbene radical cation. 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

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