Phenols, autoxidation

A variety of transition metal complexes, especially those of cobalt and copper, catalyze the oxidation of phenols. Autoxidations of 2,6-disubstituted phenols in organic solvents produce mainly the 2,6-disubstituted-1,4-benzoquinone and the 3,5,3, 5 -tetrasubstituted-4,4 -diphenoquinone with Co catalysts and certain copper catalysts (eq. Poly(2,6-... 

The method of latex synthesis for thiol autoxidation catalysts was different from that used for the phenol autoxidation catalysts. Surface active quaternary ammonium ion monomers were used. Latexes were prepared by emulsion copolymerization of 96.2 mol % styrene, 1.0 mol % divinylbenzene (technical 55% active), 0.8 mol % ethylvinylbenzene, and 2.0 mol % of monomer 4, 3 5 or 6 with azo(bisisobutyronitrile) as initiator. The conductivity of an aqueous solution of 4 before polymerization was 440 x 10" ohm l cm l. Ultrafiltration of the copolymer latex gave an initial filtrate with condutivity of 20 x 10 ohm l cm l and a... 

Talcott ST, Howard LR. Phenolic autoxidation is responsible for color degradation in processed carrot puree. J Agric Food Chem 1999 47 2109-2115. 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Diaralkyl peroxides have been prepared by autoxidation. Those compounds which autoxidize to symmetrical diaralkyl peroxides form highly stabilized radical intermediates, eg, triphenylmethane, 9-phenylanthrone, and 2,4,6-tri(/-butyl)phenol (44,66). Compounds that form stable radicals by cleavage of carbon—carbon bonds can be autoxidized to diaralkyl peroxides (69). 

The best preparative results from autoxidation are encountered when only one relatively reactive hydrogen is available for abstraction. The oxidation of isopropylbenzene (cumene) is carried out on an industrial scale, with the ultimate products being acetone and phenol ... 

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]. 

In salicylaldehyde the phenolic odour predominates. The aldehyde is much less liable to autoxidation than is benzaldehyde. 

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

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

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. 

Antioxidants are compounds that inhibit autoxidation reactions by rapidly reacting with radical intermediates to form less-reactive radicals that are unable to continue the chain reaction. The chain reaction is effectively stopped, since the damaging radical becomes bound to the antioxidant. Thus, vitamin E (a-tocopherol) is used commercially to retard rancidity in fatty materials in food manufacturing. Its antioxidant effect is likely to arise by reaction with peroxyl radicals. These remove a hydrogen atom from the phenol group, generating a resonance-stabilized radical that does not propagate the radical reaction. Instead, it mops up further peroxyl radicals. In due course, the tocopheryl peroxide is hydrolysed to a-tocopherylquinone. 

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. ... 

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). 

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. 

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