Coupling Of Phenoxy Radicals Research Articles

Formation of Pyro-products by the Pyrolysis of Monobromophenols

Thermal behavior of bromphenols was investigated by direct pyrolysis at high temperature. The thermal degradation products formed by the pyrolysis of mono-bromophenols (o-, m-, and p-) were identified by gas chromatography-mass spectrometry. During the pyrolysis reactions, several kinds of dioxins and furans were produced, and the relative ratio of pyro-products was dependent on the substituted position of bromine in phenolic structure due to the effect of symmetry and steric hindrance. The formation of dioxins can be explained by the phenoxy radical addition and Br atom elimination at an ortho-carbon site on phenolic structure. On the other hand, the formation of furans can be explained by the ortho-ortho carbon coupling of phenoxy radicals at unsubstituted sites to form o, o'-dihydroxydiphenyl intermediate via its keto-tautomer, followed by <TEX>$H_2O$</TEX> elimination. The pyrolysis temperature has also a substantial effect on the dimerized products quantities but little effect on the type of pyro-products. Moreover, the formation mechanism of pyro-products was suggested on the basis of products identified.

Wet strength improvement of unbleached kraft pulp through laccase catalyzed oxidation

Previous investigations have shown that laccase catalyzed oxidation of lignin containing wood fibers can enhance the strength of medium density fiberboards. In the present work it was investigated if laccase treatment had any impact on the tensile strength of a high yield unbleached kraft pulp. Treatment with laccase alone had only a very little effect on the wet strength of the pulp, whereas addition of lignin rich extractives increased the wet strength after the enzyme treatment significantly. A mediated oxidation gave a similar improvement of the wet tensile strength although no lignin was added to the fiber suspension. Furthermore, it was found that a heat treatment combined with a mediated oxidation gave a higher improvement in wet tensile strength than could be accounted for by the individual treatments. No change in dry tensile strength from the laccase treatment was observed. It is suggested that the observed improvement in wet tensile strength is related to polymerization of lignin on fibers in the hand sheet and/or coupling of phenoxy radicals on lignin associated to adjacent fibers. For the different mediators studied, a correlation was found between oxygen consumption upon mediated oxidation and generation of wet strength in the pulp.

Effect of para-substituents of phenolic antioxidants

The effect of substituents on phenolic antioxidants was studied extensively in the 1960s. Based on our recent study, however, there remain some unclear points in the interpretation of the functions of o- and p- substituents. In this work, a phenol having an ortho-alkoxy substituent, in particular methoxy, has been investigated in detail. Interestingly, it has been found that the introduction of a p-substituent such as an alkoxy or alkyl group develops antioxidant activity in o-methoxy phenols, although this phenol itself does not have any antioxidant activity. The p-substituent is considered to have two functions. One is the increase in the electron density on phenolic hydroxyl oxygen by the electron-donating nature of the substituent. Another is to localize the radical electron on the p-position by the inductive effect of the p-substituent. These functions of a p-substituent accelerate hydrogen abstraction by a peroxy radical from the phenol to enhance k inh and decrease the coupling of phenoxy radicals, that is, increase the n value.

ChemInform Abstract: A New Methodology for the Bis(oxocyclohexadienyl) Peroxide Formation.

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A New Methodology for the Bis(oxocyclohexadienyl) Peroxide Formation

Symmetrically substituted bis(4-oxocyclohexa-2,5-dienyl) peroxide 5 (R = R‘) as well as unsymmetrically substituted 5 (R ≠ R‘) can be prepared efficiently by treating 4-halogenocyclohexa-2,5-dienone 3 with 4-hydroperoxycyclohexa-2,5-dienone 4 in the presence of an appropriate positive halogen compound such as N-iodosuccinimide. Acetonitrile is a suitable solvent for the reaction. The formation of 5 is suggested to take place via electrophilic attack by the positive halogen species upon 3 generating the 4-oxocyclohexa-2,5-dienyl cation (or the phenoxy cation), followed by nucleophilic attack by 4 upon the cation. It is emphasized that some of the peroxides obtained by this means have not been prepared by the classical method, coupling of phenoxy radicals with O2.

Stereoselective bimolecular phenoxy radical coupling by an auxiliary (dirigent) protein without an active center.

The regio- and stereospecificity of bimolecular phenoxy radical coupling reactions, of especial importance in lignin and lignan biosynthesis, are clearly controlled in some manner in vivo; yet in vitro coupling by oxidases, such as laccases, only produce racemic products. In other words, laccases, peroxidases, and comparable oxidases are unable to control regio- or stereospecificity by themselves and thus some other agent must exist. A 78-kilodalton protein has been isolated that, in the presence of an oxidase or one electron oxidant, effects stereoselective bimolecular phenoxy radical coupling in vitro. Itself lacking a catalytically active (oxidative) center, its mechanism of action is presumed to involve capture of E-coniferyl alcohol-derived free-radical intermediates, with consequent stereoselective coupling to give (+)-pinoresinol.

Alkaline Oxygen Oxidation of Dioxane Lignin and Creosol in Aqueous Ethanol

Abstract The degradation of dioxane lignin with oxygen in alkaline aqueous ethanol was studied. An increase in the ethanol concentration resulted in an accelerated degradation of lignin. Separate experiments with creosol (2-methoxy-4-methylphenol) indicated that this effect can at least partly be ascribed to the inhibition of the coupling of phenoxy radicals, which prevents the recondensation of the lignin fragments.

Ligninase-mediated phenoxy radical formation and polymerization unaffected by cellobiose: quinone oxidoreductase

Phanerochete chrysosporium ligninase (+ H 2O 2) oxidized the lignin substructure-related compound acetosyringone to a phenoxy radical which was identified by ESR spectroscopy. Cellobiose:quinone oxidoreductase (CBQase) + cellobiose, previously suggested to be a phenoxy radical reducing system, was without effect on the radical. Ligninase polymerized guaiacol and it increased the molecular size of a synthetic lignin. These polymerizations, reflecting phenoxy radical coupling reactions, were also unaffected by the CBQase system. We conclude that ligninase catalyzes phenol polymerization via phenoxy radicals, which CBQase does not affect. The CBQase system also did not produce H 2O 2, and its physiological role remains obscure. Glucose oxidase + glucose did produce H 2O 2 as expected, but, like CBQase, it did not reduce the phenoxy radical of acetosyringone. Because intact cultures of P. chrysosporium depolymerize lignins, it is likely that phenol polymerization by ligninase is prevented or reversed in vivo by an as yet undescribed system.