Phenol hydrogen atom transfer from

Synergistic behavior by two antioxidants is not confined to compounds which inhibit by entirely different mechanisms—for example, two chain-breaking phenolic antioxidants may synergize one another. This homosynergism is caused by the suppression of the unfavorable chain propagation reactions of one phenoxy radical by a hydrogen atom transfer from the second phenol. 

Hydrogen atom transfer from phenols to radicals... 

The mechanism shown in Scheme 4.9 has been proposed for the hydrogen atom transfer from phenols (ArOH) to radicals (Y ) in non-aqueous solvents, a kinetic effect ofthe solvent (S) being expected when ArOH is a hydrogen bond donor and the solvent a hydrogen bond acceptor. Steps with mechanistic rate constants k, k-1 and k>, involve proton transfer (the latter two near to the diffusion-controlled limit), and kj involves electron transfer. The step with rate constant fco involves a direct hydrogen atom transfer, and the other path around the cycle involves a stepwise alternative. 

Scheme 4.9 Proposed mechanism for hydrogen atom transfer from phenols to radicals in non-aqueous solvents [19],...

The commonest compounds containing O—H bonds are water, alcohols, phenols and carboxylic acids (if inorganic acids and bases are neglected). Hydrogen atom transfer from water to a radical seems to be unknown (Kondratiev, 1970 Bennett et al., 1970 for deuterium atom transfer from D20 to N see Felder et al., 1970). 

Quantitative kinetic studies of absolute rate constants for hydrogen atom transfer from substituted phenols to polystyrene peroxyl radicals by Howard and Ingold in the 1960s provided the first reliable data on substiment effects on antioxidant activities of phenols. Later, a very detailed report appeared providing data on substituent and structural effects on various classes of monohydroxy phenols. In addition, detailed reviews were given of substituent etfects ". These reports provide the basis for understanding how substituent and strucmral effects control the antioxidant activities of phenols and will be summarized in part below. 

Ingold and coworkers have also expressed a kinetic equation (equation 43) to determine the equilibrium constant ( T ) for hydrogen bonding between the phenolic antioxidant and dilute HBA solvent. A, if rate constants in neat HBA and non-HBA solvent, nA, CCI4 in this example, are known. For this study they monitored hydrogen-atom transfer from the antioxidant phenol to cumyloxyl radicals . ... 

The first suggestion on the mechanism of the reaction between phenols and peroxy radicals emerged from experiments on the antioxidant effect of phenols, by Bolland and ten Have (1947a, b). These authors found a fair correlation between the increase of chain-terminating efficiency and the decrease of the redox potential of phenols and suggested a mechanism in which the phenolic hydrogen atom is transferred to the peroxy radical, i.e.,... 

Although it is established that nitrous acid and a phenoxy radical are formed in reaction sequence (Equation 5.91 and 5.92), it is unlikely that this process involves direct phenolic hydrogen-atom abstraction by NO. The alternative mechanism, which includes electron transfer from the phenoxide ion to NO2, is quite probable [41,44] ... 

Photodecomposition of some ortfeo-substituted aryldiazonium ions leads to unusual reaction products due to hydrogen atom transfer between the ortho-substituent and the radical center derived by loss of nitrogen from the diazonium ion. Thus, 2-diethylaminobenzenediazonium salts yield N-ethylaniline via the intermediate 3. Deuterium is not incorporated into the benzene ring from either DjO or CD3OD.2 Decomposition of the diazonium salt 4 leads to a benzocyclobutene as the principal product. The influence of the nitro substituent is important, because the un-nitrated diazonium salt is decomposed to give a phenol in the normal manner. ... 

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