Autoxidation of Phenols

Moussavi M. 1979. Effect of polar substituents on autoxidation of phenols. Water Res 13 1125-1128. 

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). 

The autoxidation of phenols is slow in neutral and, especially, in acid solution but becomes very noticeable in alkaline solutions. This base catalysis of phenol oxidation is of course due to the conversion of the neutral phenols to the phenolate ions, which are more easily oxidized by the oxidant Ox (equation 18) than their conjugate acids. 

Martin, J. P., Haider, K., and Bondietti, E. (1975). Properties of model humic acids synthesized by phenoloxidase and autoxidation of phenols and other compounds formed by soil fungi. Proc. Int. Meet. Humic Substances, Nieuwersluis, 1972, pp. 171-186, Pudoc, Wageningen. 

The situation outlined above is subject to dramatic changes in the presence of some transition metal ions. Therefore, as seen for many other phenolics, in the presence of small concentrations of copper or iron ions, a steep increase in the autoxidation rate for HA could be expected. The mechanisms by which copper and iron catalyse the autoxidation of phenolics do not follow the same routes [62], and so, for example in the case of 1,2,4-benzenetriol (a toxic benzene metabolite in the liver), iron(III) was found to be a comparatively inefficient autoxidation catalyst in comparison with copper(II). It is worth noting, however, that in the case of that substrate the addition of SOD shows an inhibitory effect towards autoxidation, evidently because superoxide compulsorily participates as... 

In the absence of light, metal ions, or bases, autoxidation of phenols (Scheme 14.2) is typically a slow process. 

Autoxidation of phenolic compounds may occur during extraction, particularly at alkaline pH values, but can be prevented, or at least limited, by extracting under oxygen-free nitrogen. It is advisable to treat reference compounds in a similar manner. After extraction with water, alkali or EDTA, extracts are acidified, usually to about pH 2.5, to convert the phenolics to the undissociated form. This enables them to be separated by partition into a suitable organic solvent (e.g. diethyl ether, ethyl acetate). In order to minimize the cis-trans isomerisation of the substituted... 

Autoxidation may in some cases be of preparative use thus reference has already been made to the large-scale production of phenol+ acetone by the acid-catalysed rearrangement of the hydroperoxide from 2-phenylpropane (cumene, p. 128). Another example involves the hydroperoxide (94) obtained by the air oxidation at 70° of tetrahydro-naphthalene (tetralin) the action of base then yields the ketone (a-tetralone, 95), and reductive fission of the 0—0 linkage the alcohol (a-tetralol, 96) ... 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

As described earlier, the mechanism of inhibited chain oxidation depends on the structural features of RH and InH, as well as on the reaction conditions (T, v,[RH], [InH], [O2], and [ROOH]). In this section we present data illustrating this approach with reference to the autoxidation of hydrocarbons inhibited by sterically nonhindered phenols of group A. 

Nitroxyl radicals as alkyl radical acceptors are known to be very weak antioxidants due to the extremely fast addition of dioxygen to alkyl radicals (see Chapter 2). They retard the oxidation of solid polymers due to specific features of free radical reactions in the solid polymer matrix (see Chapter 19). However, the combination of two inhibitors, one is the peroxyl radical acceptor (phenol, aromatic amine) and another is the alkyl radical acceptor (nitroxyl radical) showed the synergistic action [44-46]. The results of testing the combination of nitroxyl radical (>NO ) (2,2,6,6-tetramethyl-4-benzoylpiperidine-l-oxyl) + amine (phenol) in the autoxidation of nonene-1 at 393 K are given here ([>NO ]o + [InH]o = 1.5 x 10 4mol L 1 p02 98 kPa) [44]. 

Autoxidation. Self-catalyzed oxidation in the presence of air. Autoxidation can be initiated by heat, light, or a catalyst. The commercial production of phenol and acetone from cumene is autoxidation. Other examples include the degradation of polymers exposed to sunlight for long periods of time gum formation in lubricating oils and gasoline and the spoilage of fats. 

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

Alternatively, the fast autoxidation of mercaptans is achieved by working in basic solution and in the presence of transition metal catalysts, via reactions involving RS" anions and redox steps, much as for phenols (22). 

H. S. Olcott (University of California, Berkeley, Calif.) We have studied the effects of aliphatic amines on the autoxidation of a fish oil and squalene in air at moderate temperatures. There was little protection unless phenolic-type inhibitors were also added, in which case secondary amines were more effective than primary or secondary amines. However, at 70 °C. trioctylamine alone protected the fish oil, whereas at lower temperatures it did not (2). Further study revealed that peroxides react with trioctylamine to yield some dioctylhydroxylamine which has antioxidant properties (1). These and other observations (3) indicate that... 

From these results, it is clear that neither Equation A nor B represents the kinetics of the zinc diisopropyl dithiophosphate-inhibited autoxi-dation of cumene or Tetralin. This does not immediately indicate that the mechanism in Scheme 1 is wrong since it is highly idealized and takes no account of possible side reactions. A similar situation occurs in the inhibition of hydrocarbon autoxidation by phenols (AH), for which a basic mechanism similar to that in Scheme 1 is accepted. Termination occurs via Reactions 7 and 8 instead of Reactions 5 and 6. 

An interesting effect of Ca2+ ions on the autoxidation of hydroxy-1,4-naphthoquinone, which may have a bactericidal function in sea urchins, has been reported. Autoxidation resulted in the detection of a semiquinone, which was accompanied by only minimal oxygen uptake. The presence of Ca2+ resulted in a massive enhancement in the rate of 02 uptake. Potentiometric titrations revealed that Ca2+ ions, by forming complexes with the compound, lowered the pKa values of its phenolic groups, thereby generating the oxidation-prone phenolate anions. Autoxidation was also facilitated by the spin stabilisation effect of the Ca2+ ions on the naphthosemiquinone.121 This effect may be of importance in mammalian cells, where quinones induce elevations in the free Ca2+ concentration. 

The ready formation of benzylic hydroperoxides is used in industrial oxidations, as in the synthesis of propylene oxide and phenol (see Sections 9.5.2 and 9.5.4, respectively). In contrast with autoxidation of alkenes, where various secondary processes may follow, autoxidation of arenes is less complicated. Chain termination of 99 may lead to an alcohol and aldehyde [Eq. (9.151)], and the rapid autoxidation of the latter may produce the corresponding carboxylic acid [Eq. (9.152)] ... 

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