Stoichiometric reactions, phenoxyl radical

In studies of the reactivity of copper-phenoxyl radical species, particular emphasis has been placed on performing alcohol oxidations, either stoichiometrically or catalytically, and by using O2 as the final oxidant. One goal has been to obtain mechanistic insights into GAO function. In addition, the GAO reaction sequence has been used as inspiration for the development of new chemical catalysts for alcohol oxidations and H2O2 generation (a bioinorganic success story ). ... 

The stoichiometric factors of inhibition and the rate constants of the ter-penephenols (TP) with isobornyl and isocamphyl substituents were determined by the reaction with peroxy radicals of ethylbenzene. The reactivity was found to decrease for o-alkoxy compared with o-alkyl substituent caused by the intramolecular hydrogen bond formation that is conformed by FTIR-spectroscopy. The inhibitory activity for mixtures of terpene-phenols with 2,6-di-ferf-butyl phenols in the initiated oxidation of ethylbenzene was also studied. In spite of the similar antiradical activities of terpenephenols with isobornyl and isocamphyl sunstituents, the reactivity of phenoxyl radicals formed from them are substantially different that is resulted from the kinetic data for mixtures of terpenephenols with steri-cally hindered phenols. 

Unlike the peroxidation of the hydrocarbon polymers, the oxidation of lignin occurs by a stoichiometric process and not a chain reaction. Because phenols are antioxidants, the phenoxyl radicals formed are too stable to participate in a peroxidation chain reaction and the aromatic system is converted to quinoid compounds and ultimately humus. Both abiotic transition metal ion catalysed peroxidation and biological oxidation ire involved in the conversion of lignin to hiunus. 

The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and amin-oxyl from aromatic amines) cannot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. It should be noted that this process is stoichiometric and hydroperoxides are produced in each inhibiting step (reaction 10). 

The chemical mechanisms involved in the action of antioxidants have been discussed in a number of reviews [8,11-18] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor process, (CB-D) summarised in reaction (3). The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and aminoxyl from aromatic amines) carmot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. However, it should be noted that this process is stoichiometric and hydroperoxides... 

---