Induced-radical mechanism

As already discussed in Section 2.2.1, perfluoroalkyl halides do not act as effective electrophilic perfluoroalkylation reagents, as might be expected by analogy with the reactivity of alkyl halides. Even if some electrophilic perfluoroalkylation reactivity is mimicked with some especially suitable (i. e. easily oxidizable) substrates by an electron-transfer-induced radical mechanism, the practical usefulness of this reaction pathway is limited to very few examples. 

There are three significant variations on the Favorskii rearrangement, the homo-, quasi-, and photo-Favorskii rearrangements. The homo-Favorskii and quasi-Favorskii rearrangements oecur if the precursor does not possess the classic a-hydrogen and a -halide. The photo-Favorskii is a variation involving a light-induced radical mechanism. 

Certain electrophilic species such as CI2, Brg, and BrNg can add to olefins by ionic or molecule-induced radical mechanisms depending on the conditions. Under radical-inhibiting conditions, methyl hypobromite adds to hex-l-ene to yield primarily (348), but (349) is obtained under radical-stimulating conditions. Acetyl hypobromite added to hex-l-ene exclusively... 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

Sadrzadeh, S.M.H. and Eaton, J.W. (1992). Hemt obin-induced oxidant damage to the central nervous system. In Free Radical Mechanisms of Tissue Injury (eds. M.T. Moslen and C.V. Smith) pp. 24—32. CRC Press, Boca Baton. 

Nordmann, R., Ribiere, C. and Rouach, H. (1992). Implication of free radical mechanisms in ethanol-induced cellular injury. Free Rad. Biol. Med. 12, 219-240. 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

This division is maintained in this article. However, it is not necessarily a mechanistic division, since thermally induced reactions may also proceed by a radical mechanism. 

This kind of reaction can proceed in solution by a carbocation mechanism, but in the radiation-induced case, it proceeds almost exclusively by a radical mechanism. In most cases, the radiation initiates reactions that are of chain character. 

Radiation-induced substitution reactions have been reviewed by Wilson (1972) with examples of nitration, nitrosation, sulfochlorination, and others. These generally proceed by a free-radical mechanism. The free radicals are generated by the action of radiation on the reagent, which is present in large excess—for example,... 

On the other hand, in accord with the free radical mechanism peroxynitrite is dissociated into free radicals, which are supposed to be genuine reactive species. Although free radical mechanism was proposed as early as in 1970 [111], for some time it was not considered to be a reliable one because a great confusion ensued during the next two decades because of misinterpretations of inconclusive experiments, sometimes stimulated by improper thermodynamic estimations [85]. The latest experimental data supported its reliability [107-109]. Among them, the formation of dityrosine in the reaction with tyrosine and 15N chemically induced dynamic nuclear polarization (CIDNP) in the NMR spectra of the products of peroxynitrite reactions are probably the most convincing evidences (see below). 

These results, as well as rate studies " and kinetic isotope effects ", support a concerted, 5ptra-structured oxenoid-type transition state for the CH oxidations". The original oxygen-rebound mechanism has been discounted (see the computational work in Section I.D). Recently, however, the stepwise radical mechanism was revived in terms of the so-called molecule-induced homolysis , but such radical-type reactivity was severely criticized on the basis of experimental" and theoretical grounds. 

Oxidation by molecular oxygen most likely occurs via a radical mechanism 10-13 and the reaction rates are generally slow unless traces of metal ions are present which are known to drastically affect the reaction rate 14 thus making control of the air-oxidation reaction rather difficult. The rates can also be significantly enhanced by adding charcoal to induce a surface-assisted catalysis of the intramolecular disulfide bond formation 15 Nonetheless the difficult control of this oxidation procedure can lead to partial oxidation of Met and Trp residues when peptides are exposed for longer periods of time to air oxygen 16 ... 

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