Mechanism of the oxidation of 2,6-di-tert-butylphenols containing electron-donor substituents in the para-position
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The formation of 2,6-di-tert-butyl-1,4-benzosemiquinone is characteristic of the oxidation of 2,6-di-tert-butylphenols containing electron-donor substituents in the para-position by molecular oxygen in an alkaline aqueous alcohol solution.
Some products of the oxidation of 4-substituted 2,6-di-tert-butylphenols in alkaline aqueous alcohol solution were isolated and identified.α-N, N-Dimethylamino- andα-methoxymethyl-2,6-di-tert-butyl-methylenequinones, formed from 4-N,N-dimethylamino- and 4-methoxymethyl-2,6-di-tert-butylphenols, respectively, are of special interest; the previously undescribed 3-hydroxy-2,6-di-tert-butyl-1,4-benzo-quinone was isolated from the oxidation products of 4-(β-ethoxy)aminomethyl-2,6-di-tert-butylphenol; the structure of this product was demonstrated by IR, EPR, and NMR spectroscopy. This hydroxyquinone is formed in the direct reaction of 2,6-di-tert-butyl-1,4-benzoquinone with alkali,
The addition of 2,6-di-tert-butyl-1,4-benzoquinone during the oxidation of 2,6-di-tert-butyl-4-aminoalkylphenols in an alkaline aqueous alcohol solution produces a sharp increase in the concentration of semiquinone ion radicals, which is associated with the reduction of the quinone in alkaline medium. The addition of oxygen during oxidation probably leads to an increase in the quinone concentration in the system; this causes a subsequent increase in the concentration of semiquinone radicals. Possible schemes of reduction of the quinone in alkaline solution are discussed.
A general scheme is proposed for the oxidation of sterically hindered phenols with electron-donor substituents in the para-position by molecular oxygen in polar media.
KeywordsOxidation Phenol Quinone Oxidation Product Molecular Oxygen
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