Autoxidation hydroquinones

Catalyzed autoxidation. Hydroquinones and benzoquinone dioximes are converted to quinones and dinitrosobenzenes, respectively, on autoxidation in the presence of NO2. 

Vinyl acetate is normally inhibited with hydroquinone to prevent polymerisation. A combination of too low a level of inhibitor and warm, moist storage conditions may lead to spontaneous polymerisation. This process involves autoxidation of acetaldehyde (a normal impurity produced by hy droly sis of the monomer) to a peroxide which initiates exothermic polymerisation as it decomposes. In bulk, this may accelerate to a dangerous extent. Other peroxides or radical sources will initiate the exothermic polymerisation. 

Since the substituted hydroquinones and quinone dioximes are better electron donors than hexamethylbenzene (as established by cyclic voltammetric studies), donor-induced disproportionation (to generate NO+ NOf) is even more favored. Furthermore, either two successive one-electron oxidations of hydro-quinone (or quinone dioxime) by NO + followed by the loss of two protons from the dication or two sequential oxidation/deprotonation steps complete the oxidative transformation in equation (97). Importantly, the ready aerial oxidation of NO to NO provides the basis for the nitrogen oxide catalysis of hydroquinone (or quinone dioxime) autoxidation as summarized in Scheme 26. 

Moreover, the efficiency of catalytic autoxidation in Scheme 26 is also attributed to the short lifetime of the hydroquinone cation radical (t < 10 10 s-1)254, which renders the sequential electron-transfer/proton-trans-fer cycles extremely efficient. 

The early practical application of antioxidants was connected with the development of rubber production. The rubber is easily oxidized in air, and the first antioxidants were empirically found and used to stabilize it [1]. Empirical search for antioxidants was performed by Moureu and Dufresse [2] during the First World War. These researchers successfully solved the problem of acrolein stabilization by the addition of hydroquinone. They explained the retarding action of the antioxidant in the scope of peroxide conception of Bach and Engler (see Chapter 1). They proposed that the antioxidant rapidly reacts with the formed hypothetical moloxide and in such a way prevents the autoxidation of the substrate. 

Commercially available 2-ethylhexanal (Eastman practical grade) is purified by fractional distillation b.p. 163-163.2°/atm., no 1.4133. Other aldehydes are conveniently prepared by the Rosenmund reduction.2 If the aldehyde is relatively unstable toward autoxidation,3 a catalytic amount (0.5-1.0 g.) of hydroquinone is added with the aldehyde-bromo ester solution. 

Antioxidant. Substances that retard or inhibit autoxidation at moderate temperatures and pressures. Commonly Icnown, commercial antioxidants are aromatic amines, alkylated phenols, cresols, and hydroquinones. 

Chemical/Physical. Ozonolysis products reported are p-quinone and dibasic acids (Verschueren, 1983). Moussavi (1979) studied the autoxidation of hydroquinone in slightly alkaline (pH 7 to 9) aqueous solutions at room temperature. The oxidation of hydroquinone by oxygen followed first-order kinetics that yielded hydrogen peroxide and / -quinone as products. At pH values of 7.0, 8.0, and 9.0, the calculated half-lives of this reaction were 111, 41, and 0.84 h, respectively (Moussavi, 1979). 

Since autoxidations of phenols and aromatic amines are non-chain radical processes, they require some rapid radical-generating step. In a few systems—e.g., hydroquinone autoxidation, this is supplied by a direct redox reaction with oxygen (11). 

Eyer, P. (1991) Effects of superoxide dismutase on the autoxidation of 1,4-hydroquinone. Chem.-... 

Many antioxidants quoted as potential protective agents against free-radical-induced DNA damage have more than one phenolic group. Their chemistry is, therefore, also of some interest in the present context. The semiquinone radicals, derived from hydroquinone by one-electron oxidation or from 1,4-benzoqui-none by one-electron reduction, are in equilibrium with their parents (Roginsky et al. 1999), and these equilibria play a role in the autoxidation of hydroquinone (Eyer 1991 Roginsky and Barsukova 2000). Superoxide radials are intermediates in these reactions. 

It has been apparent for some time that the inhibition of sulfite autoxidation by organic compounds is due to their reactions with free radical intermediates. Further, it seems likely that the chemical effects associated with S02 autoxidation are due to these radicals or, possibly, peroxymonosulfate. Now that the reactivities of the likely free radical intermediates are known, the mechanism of these effects can be begun to be understood. For many organic compounds, like hydroquinone and other phenolic species, reaction with S03 and SO " is possible. Indeed, they prove to be the most efficient inhibitors of S02 autoxidation and the order of... 

Rao (1991) showed that hematin catalyzed the autoxidation of hydroquinone or 1,2,4-benzenetriol in vitro, producing reduced oxygen species that may be responsible for protein or DNA binding after benzene exposure. Further work along this line of research has provided evidence that in vitro, chelates of iron and hydroquinone or 1,2,4-benzenetriol are potent DNA cleaving agents (Rao 1996 Singh et al. 1994), and that 1,2,4-benzenetriol, but not hydroquinone, causes the release of iron from ferritin (Ahmad et al. 1995). 

Rao GS. 1991. Hematin catalyzed autoxidation of hydroquinone or 1,2,4-benzenetriol. Chem Biol Interactions 80 339-347. 

Commercial benzaldehyde inhibited against autoxidation with 0.1% hydroquinone is usually satisfactory. If the material available is yellow or contains benzoic acid crystals it should be shaken with equal volumes of 5% sodium carbonate solution until carbon dioxide is no longer evolved and the upper layer dried over calcium chloride and distilled (bp 178-180°C), with avoidance of exposure of the hot liquid to air. The distillation step can be omitted if the benzaldehyde is colorless. 

During the inhibited self-initiated autoxidation of methyl linoleate by a-Toc in solution, Niki and coworkers made the interesting observation that a-Toc acts as an antioxidant at low concentrations, but high concentrations (up to 18.3 mM) actually increased hydroperoxide formation due to a pro-oxidant effect. The pro-oxidant effect of a-Toc was observed earlier by Cillard and coworkers in aqueous micellar systems and they found that the presence of co-antioxidants such as cysteine, BHT, hydroquinone or ascor-byl palmitate inverted the reaction into antioxidant activity, apparently by reduction of a-To" to a-Toc . Liu and coworkers ° found that a mixture of linoleic acid and linoleate hydroperoxides and a-Toc in SDS micelles exhibited oxygen uptake after the addition of a-Toc. The typical ESR spectrum of the a-To" radical was observed from the mixture. They attributed the rapid oxidation to decomposition of linoleate hydroperoxides, resulting in the formation of linoleate oxy radicals which initiated reactions on the lipid in the high concentration of the micellar micro-environment. Niki and coworkers reported pro-oxidant activity of a-Toc when it was added with metal ions, Fe3+25i Qj. jjj (jjg oxidation of phosphatidyl choline liposomes. a-Toc was found... 

The autoxidation of hydroquinone is accompanied by the fonnation of the p-semiquinone radical this was shown as early as 1938 by Michaelis and coworkers Since then numerous additional examples of this type of reaction have been described, relating not only to hydroquinones but also to catechols, resorcinols, pyrogallols, naphthols and substituted phenols. Most of this material has been reviewed including that relating to synthetic application of phenol oxidation and to phenoxyl radicals involved in the biosynthesis of natural products -... 

In a few cases, catechols and hydroquinones have been found to undergo further oxidation to quinones (reactions 1-G and l-I). Such reactions occur by two single-electron steps and can be either enzymatic or nonenzy-matic (i.e., resulting from autoxidation and yielding as by-product the superoxide anion-radical 0, -). The intermediate in this reaction is a semiquinone. Both quinones and semiquinones are reactive, particularly to-... 

Thus, a,a- and /J,/9-dinaphthylamines, and hydroquinone markedly check autoxidation of paraffins at 160° C. but lead tetraethyl and other antiknocks have but little effect.89 Layng and Youker found that in comparison with lead tetraethyl in the gas phase, diphenylamine had a hundred times the effectiveness as an oxidation inhibitor as it possessed as an antiknock dope. Some distinction evidently must be made between inhibitors which act at low or moderate temperatures and antiknock dopes which alter combustion phenomena but do not affect low temperature oxidation. Moureu, Dufraisse and Cliaux explain the ineffectiveness of lead tetraethyl at low temperatures as due to extreme ease of oxidation but this has not been demonstrated. On the contrary it has been shown that the effective antiknock dopes are not readily oxidized by air.01... 

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