Phenoxy 1 radicals

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

The halogen displacement polymerization proceeds by a combination of the redistribution steps described for oxidative coupling polymerization and a sequence in which a phenoxide ion couples with a phenoxy radical (eq. 11) and then expels a bromide ion. The resultant phenoxy radical can couple with another phenoxide in a manner that is analogous to equation 11 or it can redistribute with other aryloxy radicals in a process analogous to equations 7 and 8. 

Spin density for phenoxy radical shows location of unpaired electron. 

Repeat your analysis fox phenoxy radical. Instead of charge, focus on the spin density. Calculate the delocalization energy using phenoxy radical at phenol geometry. Is it of the same order of magnitude as that for phenoxy anion Explain. 

The process is exothermic, suggesting that the phenoxy radical is particularly stable. Display the spin density surface for phenoxy radical. Is the unpaired electron localized or delocalized over several centers Is the unpaired electron in the a or 7t system Draw appropriate Lewis structures that account for your data. 

Aryl peroxyesters are generally unsuitable as initiators of polymerization owing to the generation of phenoxy radicals that can inhibit or retard polymerization... 

It should be pointed out that not all benzoin derivatives (75) are suitable for use as photoinitialors. Benzoin esters (75, R=aeyl) undergo a side reaction leading to furan derivatives. Aryl ethers (75, R=aryl) undergo (3-seission to give a phenoxy radical (an inhibitor) in competition with a-scission (Scheme 3.54). Benzoin derivatives with a-hydrogens (75 R-Il) are readily autoxidized and consequently can have poor shelf lives. 

Studies with simple radicals show that carbon-centered radicals react with phenols by abstracting a phenolic hydrogen (Scheme 5.14). The phenoxy radicals may then scavenge a further radical by C -C or C-O coupling or (in the case of hydroquinones) by loss of a hydrogen atom to give a quinone. The quinone may then react further (Section 5.4.4). Thus two or more propagating chains may be terminated for every mole of phenol.I9j... 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

This scheme is selfexplanatory. Important in the formation of PBDD by Ullmann reaction of brominated phenoxy radicals. Influences of added water, air content and other factors on PBDD/F yield and pattern have also been studied in detail (ref. 11). 

Lyons and coworkers studied the ESR spectra of bakelite polysulfone [—CgH4— O—CgH4—SO2—CgH4—O—CgH4—C(CH3)2—] y-irradiated at 77 K and found features characteristic of at least four radicals, the cyclohexadienyl radical, formed from addition to the aromatic ring, methylene groups (— CH2) formed from H abstraction from the methyl group, phenoxy radicals and peroxy radicals. 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

The oxidation generates highly delocalized phenoxy radicals (PhO, Scheme 2.21), which may initiate (i) a radical polymerization process, trapping the reactant (CF) to give a benzyl radical intermediate (QMR), or it may (ii) follow a radical coupling to produce the p-QM p-O-QM, which being a reactive electrophile could undergo cationic polymerization. 

SCHEME 10.5 Proposed pathway for the nonenzymatic conversion of BHTOOH to BHT-QM in keratinocytes. BHTOOH is oxidized to a peroxy radical that spontaneously loses oxygen. Two BHT phenoxy radicals then undergo disproportionation. 

Lloyd, R. V. Wood, D. E. Free-radicals in an adamantane matrix. 8. EPR and INDO study of benzyl, anilino, and phenoxy radicals and their fluorinated derivatives. J. Am. 

The trityl radical (gold-coloured) is readily oxidized to peroxide (white) the comparable 2,4,6-tri-(tert-butyl)phenoxy radical (blue) in, e.g., cyclohexane was applied by Paris et al." to so-called free radical titration (either potentiometric or photometric) of oxygen or antioxidant (the latter by hydrogen abstraction). 

The recognition2 that the a, p-dimer 3 is formed in equilibrium with 1 and not the a,a-dimer 2 was interpreted as a result of the smaller steric strain in 3 than in 23 Also the known strong influence of p-substituents on the equilibrium constants between substituted trityl radicals and their dimers6 found an obvious explanation in this way. The earlier observation that not only those phenoxy radicals 4 carrying three conjugating phenyl substituents 4 (R = C6H5)7a are persistent8 but also their... 

The radicals (14) formed may be trapped with, for example, (10) above. Simple alkyl thiyl radicals such as MeS have been detected as reaction intermediates they are highly reactive. Relatively stable oxygen-containing radicals are also known. Thus the phenoxy radical (15),... 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

Phenolic compounds may enhance the rate of decomposition of aromatic ether, because the phenoxy radical may be stabilized by solvation (18) or hydrogen bonding (19) with phenolic compounds and may result in the subsequent hydrogen transfer reaction from hydrogen donating solvent or phenols (20). 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

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