Autoxidation, initiation

Functional groups that stabilize radicals would be expected to increase susceptibility to autoxidation. This is illustrated by two cases that have been relatively well studied. Aldehydes, in which abstraction of the aldehyde hydrogen is fecile, are easily autoxidized. The autoxidation initially forms a peroxycarboxylic acid, but usually the corresponding carboxylic acid is isolated because the peroxy acid oxidizes additional aldehyde in a... 

Figure 18.2. Cholesterol autoxidation initiated by peroxy (LOO ) or alkoxyl (LO ) radicals arising from peroxidation of polyunsaturated fatty acids (LH). Compounds are as follows (1) CHOL (2) 7-CHOL (3) CHOO (4) 7a-OOH (5) 7fLOOH (6) CHO (7) 7a-OH (8) 7(3-OH (9) 7-keto (10) 7-keto-3,5-dien. For abbreviations, see Table 18.1.

Rudzinski K.J. and W. Pasiuk-Bronikowska Isoprene inhibition of S(IV) autoxidation initiated by peroxydisulphate, in P.M. Midgley, M. Reuther, M. Williams (eds). Transport and chemical transformation in the troposphere. Proc. EUROTRAC-2 Symposium 2000, Springer Verlag, Berlin (2001) 1-4 (CD). 

Formation and reaction of radicals are closely linked. The general scheme of lipid autoxidation - initiation, propagation, and termination -provides an outline of typical radical reactions. 

Ozone is known to react with virtually all organic materials, the rate of reaction being considerably faster with certain unsaturated compounds than with saturated materials. In the latter case it is probable that ozone attack is associated with an autoxidation initiation process. In addition, certain reactions of ozone with organic materials may provide a source of singlet oxygen. It is however in unsaturated materials that ozone has the greatest effect and it is with this aspect that this section is concerned. 

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

AlkoxyaLkyl hydroperoxides are more commonly called ether hydroperoxides. They form readily by the autoxidation of most ethers containing a-hydrogens, eg, dioxane, tetrahydrofuran, diethyl ether, diisopropyl ether, di- -butyl ether, and diisoamyl ether (10,44). From certain ethers, eg, diethyl ether (in the following, R = H R = 35 — CH2CH2), the initially formed ether hydroperoxide can yield alcohol on standing, or with acid treatment... 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

Most polymer degradation caused by the absorption of uv light results from radical-initiated autoxidation. 

Table 7.4 Reactions responsible for initiation of autoxidation. (After Scott )...

The fimction of an antioxidant is to divert the peroxy radicals and thus prevent a chain process. Other antioxidants fimction by reacting with potential initiators and thus retard oxidative degradation by preventing the initiation of autoxidation chains. The hydroperoxides generated by autoxidation are themselves potential chain initiators, and autoxidations therefore have the potential of being autocatalytic. Certain antioxidants fimction by reducing such hydroperoxides and thereby preventing their accumulation. 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

The common initiators of this class are f-alkyl derivatives, for example, t-butyl hydroperoxide (59), Aamyl hydroperoxide (60), cumene hydroperoxide (61), and a range of peroxyketals (62). Hydroperoxides formed by hydrocarbon autoxidation have also been used as initiators of polymerization. 

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

The first term is identical with that observed for autoxidation of U(IV) and a similar mechanism involving hydrolysis is probable for the initial stage, although the authors prefer an initial three-equivalent oxidation of Ti(Ill) to give Ti(Vl), OH- and OH . 

The general reluctance of reagents to reduce O2 in one-equivalent states is further exemplified in the cases of V(II) and Cr(II). Autoxidation of V(1I), ultimately to V(III), produces ° an intermediate dimer, VOV ", identified spectrally and a known product of the reaction between V(II) and Clearly one path involves an initial two-equivalent oxidation of V(ri) to V(IV) and Swinehart ° calculates that 60 % of the oxidation follows this route. Cr(Il) perchlorate produces a species containing two Cr(III) species linked by one 0x0-or two hydroxo-bridges and Cr(lV) is proposed as the first intermediate F... 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

In conclusion, oxidation of carotenoids by molecular oxygen, the so-called autoxidation process, is a complex phenomenon that is probably initiated by an external factor (radical, metal, etc.) and for which different mechanisms have been proposed. The autoxidation of a carotenoid is important to take into account when studying antioxidant activity because it can lower the apparent antioxidant activity of a carotenoid. ... 

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

Generation of a radical through an oxidative process probably occurs in the initiation of the autoxidation of benzaldehyde (p. 319), which is catalysed by a number of heavy metal ions capable of one-electron transfers, e.g. Fe3 ... 

Aldehydes, and particularly aromatic ones, are highly susceptible to autoxidation thus benzaldehyde (97) is rapidly converted into benzoic acid (98) in air at room temperature. This reaction is catalysed by light and the usual radical initiators, but is also highly susceptible to the presence of traces of metal ions that can act as one-electron oxidising agents (cf. p. 306), e.g. Fe3 , Co3 , etc ... 

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). 

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