Radicals phenol

Acceptors of peroxyl radicals (phenols, hydroquinones, aromatic amines) retard hydrocarbon oxidation, terminating the chains (see Part II). 

A mixture of two antioxidants, one acceptor of the peroxyl radical (phenol or amine) and another alkyl radical acceptor (stable nitroxyl radical), causes the synergistic effect in autoxi-dation of hydrocarbons (ethylbenzene and nonene-1) [44-46]. 

Hydroxyl radicals were generated radiolytically in NaO-saturated aqueous solutions of thiourea and tetramethylthiourea. Conductometric detection showed that HO and a dimeric radical cation were produced. The dimeric radical cation is formed by addition of a primary radical to a molecule of thiourea. In basic solution, the dimeric radical cation decays rapidly to a dimeric radical anion, which is formed via neutralization of the cation and subsequent deprotonation of the neutral dimeric radical (Scheme 16). This was not observed in tetramethylurea. These dimeric radical cations of thiourea and tetramethylurea are strong oxidants and readily oxidize the superoxide radical, phenolate ion, and azide ion. 

All unusual non-radical phenolic coupling has been observed in 4-substituted-2,6-diiodophenols.43 It is likely that the reaction which results in the liberation of iodine involves. S n2 attack by an ambident phenolate anion (5) on the a-iodo keto tautomer (6). 

The mnlticopper oxidases couple the one-electron or two-electron oxidation of their substrates to the four-electron rednction of dioxygen to water (36). The reaction with substrate can proceed via an onter-sphere or an inner-sphere mechanism, and as a resnlt, the snbstrate specificity varies substantially among the enzymes. The best-characterized enzymes are laccase, ascorbate oxidase, and cernloplasmin. Radical phenol and amine species formed by laccase and ascorbate oxidase... 

MetaDrug also includes 89 rules to predict likely reactive metabolites such as quinones, aromatic and hydroxyl amines, acyl glucuronides, acyl halides, epoxides, thiophenes, furans, phenoxyl radicals, phenols, and aniline radicals. Molecules with reactive groups are marked and highlighted. 

The redox potential of interest to understand the biological effects of flavan-3-ols is the one related to phenoxyl radical-phenate couple, as this potential is roughly 1 V lower than the potential of the phenoxyl radical-phenol couple, which furthermore may transiently involve the oxidation of the aromatic atoms. Standard potential can be measured by electrochemistry [49] or pulse radiolysis [40 4]. However, determining the redox potential of polyphenolic compounds is a real challenge since for these methods the measurement must be faster than the subsequent reactions induced by the oxidation of the phenol group in order to obtain the thermodynamic value. By using ultramicroelectrodes (electrodes with a micrometer diameter), it has been shown that a very high scan rate, up to 1 milUon... 

Hydroxyl radicals, generated radiolytically in N2O, react with thiourea and tetram-ethylthiourea to afford a radical cation. Initially an adduct forms between thiourea (or tetramethylthiourea) and HO this is followed by loss of HO and reaction of the resulting monomeric radical cation with a molecule of thiourea (or tetramethylthiourea) affords the dimeric radical cation. In the bimolecular decay of the dimeric radical cation formamidine (or tetramethylformamidine) disulfide is formed. In basic solutions of thiourea (but not tetramethylthiourea), the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical, is observed. The reactions of the dimeric radical cation with O2, superoxide radical, phenolate ion, and azide ion have also been studied. ... 

Reduction potentials have also been determined for para-substituted phenyl thiyl radicals from equilibration with phenoxyl radical/phenolate couples in equilibrium (36) and with other standards [37]. 

Lastly, norbelladine (top of Scheme 1.7) is issued from the reductive amination of 3,4-dihydroxybenzaldehyde (derived from phenylalanine) with tyramine (derived from tyrosine) and constitutes a biosynthetic node leading to Amaryllidaceae alkaloids such as galantamine, crinine, or lycorine depending on the topology of phenolic couplings. In all these biosynthetic routes, radical phenolic couplings are key reactions for C—C and C—O bond formations and rearrangements [30, 31]. 

It is however complicated to interpret spectra of thermally and/or radio-chemically aged PE samples. By-products generated from radical + phenol reaction are generally conjugated structure of which UV absorption partially overlaps with signal of xmreacted stabilizers. 

Besides their function as efficient H-donors, phenols are able to participate at other reaaions which depends on the substitution pattern phenols with 2-, 4-, and 6-tert-alkyl substitution where no tautomeric benzyl radical can be formed can react stoichiometrically with peroxy radicals. Phenols with methylene or methyl substitution at least in 2-, 4-, or 6-position are able to form corresponding quinone methides. Subsequent inter- and intramolecular recombinations often lead to generally irreversibly formed C-C coupling produas (5). 

Formation of free radicals Phenol or aniline substituted with an electron releasing group (alkoxy, hydroxyl, more than one alkyl group)... 

The first attack of the reagent is a direct oxidation by the Cu+ complex abstracting an electron and giving rise to free radicals which dimerize (Fig. 15). Figure 15 shows the reaction with naphthol and the influence of substituents on the subsequent reactions of the free radical. Phenol or naphthol free radicals do not easily react with oxygen. They rather undergo homolysis. Naphthoquinones are formed in a further... 

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