Dear Researcher,

When phenols are oxidized to form quinones, the reaction involved is a reversible reaction. Upto the stage of quinone the process is reversible. But if the oxidation proceeds, it connects the individual phenol molecules between themselves and results in an oxidative condensation with the formation of polymeric products. At this later stage of molecular condensation, the antioxidant potential of the individual moeities is lost.

In the case of a radical process, phenols react with peroxyl radicals, to give a phenoxyl radical. This one can in principle react with another POO° radical to give a cyclohexadienone or sometimes react by dismutation to regenerate a phenol + a quinone.

In my mind, once phenols have reacted to give a phenoxy or a quinone, they have lost their antioxidant properties because quinone can not give another labile hydrogen to POO° and can not trap another radical.

regards

PS: some of my papers on that subject:

E. Richaud. European Polymer Journal 49, 2223 (2013)

E. Richaud et al. Polymer Degradation and Stability 96, 1 (2011)

E. Richaud. Radiation Physics and Chemistry 103, 158 (2014)E. Richaud et al. Journal of Applied Polymer Science 110, 3313 (2008)

Thanks so much..!

Can I get more references for that? 

in my papers, they are given

 Dear Sel,

Caffeic acid, an extensively studied dietary cinnamic acid derivative, possesses vicinal hydroxyl groups; it undergoes reversible oxidation to initially yield o-quinone, as Pallavi mentioned. This is the initial step in the oxidation of o-diphenols, which can be performed by enzymes, alkaline conditions and oxidizing agents. I presume that by oxidation of caffeic acid (“solution of caffeic acid”) you are referring to autoxidation or oxidation catalyzed by oxidants. Caffeic acid is soluble in organic solvents and hot water. Caffeic acid in certain instances is dissolved using a carrier solution (by dissolving ammonium acetate in water/methanol), and suitably adjusting the pH of the sample solution.

When caffeic acid is oxidized by polyphenol oxidase (tyrosinase) or KMnO4 the first oxidation product formed is the o-quinone of caffeic acid, which is not very stable. Initial oxidation is followed by the generation of caffeic acid oligomers (e.g., tetramers).  Non-enzymic oxidation of caffeic acid occurs by autoxidation and alkaline medium favors this. Autoxidation is dependent on pH. Caffeic acid undergoes reversible oxidation in solutions with a certain pH upper limit. Initial oxidation herein is an o-quinone. After the generation of the quinone, subsequent reactions are the same as those catalyzed by polyphenoloxidase. Typically, a phenol undergoes 1-electron oxidation to generate a semiquinone, which is converted to a quinone. Oxidative coupling (-C-C) of phenols (coupling reaction of the semiquinone radical, an intermediate of one-electron oxidation) is ubiquitous, and generates dimers, which are susceptible to further polymerization. Radical scavenging activity of certain caffeic acid polymers has been assessed. Some reports indicate that these polymeric molecules display much lower radical scavenging potentiality in comparison with that of caffeic acid. 

I agree with above reply

In general quinones are not antioxidants. However, quinones could be reduced back to phenols by another antioxidant. The best example is vitamins E and C. Vitamin E after oxidation can be regenerated by ascorbic acid.

The first should be identified structure of quinones then on the interaction between functional groups recognize antioxidant power. that is clear first groups of the phenolic have more than antioxidant power ratio different quinones 

Please refer the attached file as an additional information

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