Autoxidation hydroperoxide formation

The complex and incompletely understood phenomena of cool flames and then-close relationship with autoignition processes is discussed in considerable detail. As the temperature of mixtures of organic vapours with air is raised, the rate of autoxidation (hydroperoxide formation) will increase, and some substances under some circumstances of heating rate, concentration and pressure will generate cool flames at up to 200° C or more below their normally determined AIT. Cool flames (peroxide decomposition processes) are normally only visible in the dark, are of low temperature and not in themselves hazardous. However, quite small changes in thermal flux, pressure, or composition may cause transition to hot flame conditions, usually after some delay, and normal ignition will then occur if the composition of the mixture is within the flammable limits. 

The complex and incompletely understood phenomena of cool flames and their close relationship with autoignition processes is discussed in considerable detail. As the temperature of mixtures of organic vapours with air is raised, the rate of autoxidation (hydroperoxide formation) will increase, and some substances under some... 

The duration of the inhibition period of a chain-breaking inhibitor of autoxidation is proportional to its efficiency. Indeed, with an increasing rate of chain termination, the rates of hydroperoxide formation and, hence, chain initiation decrease, which results in the lengthening of the induction period (this problem will be considered in a more detailed manner later). It should be noted that when initiated oxidation occurs as a straight chain reaction, the induction period depends on the concentration of the inhibitor, its inhibitory capacity, and the rate of initiation, but does not depend on the inhibitor efficiency. 

The autoxidation of polyunsaturated fatty acids (cf. Porter et al. 1981) is usually monitored by the formation of malonaldehyde using the 2-thiobarbituric acid essay. This is carried out under rather severe conditions which decomposes its precursor. This malonaldehyde-like product is obviously formed via a cycliza-tion reaction of a peroxyl radical, followed by other processes such as further cyclization and hydroperoxide formation [reactions (21)-(23)]. The resulting hydroperoxides may eliminate malonaldehyde upon a homolytic cleavage of the endoperoxidic intermediate (Pryor and Stanley 1975). 

The first reaction (346) consists of hydroperoxide formation by a typical autoxidation process, and the second represents selective epoxidation by the hydroperoxide. In the absence of the autoxidation catalyst, no reaction is observed under these conditions due to efficient removal of chain-initiating hydroperoxide molecules by reaction (347). Optimum selectivities obtain when the autoxidation catalyst is of low activity, which implies a low total activity of the catalytic system. The molybdenum complexes related to Mo02(oxine)2 are among the most effective catalysts for epoxidation.496 Although the autoxidation catalysts were limited to two types (phosphine complexes of noble metals and transition metal acetylacetonates), there is no reason, a priori, why other complexes such as naphthenates should not produce similar results. 

Discussion. In the autoxidation of organic compounds that takes place through the mechanism of hydroperoxide formation, one finds that following the induction stage steady state mathematics can be applied to account for the observed dependence of the rate of oxidation upon the... 

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 light emission from autoxidizing organic materials can arise from self-reaction of primary or secondary alkylperoxyl or alkoxyl radicals. In the present case, the predominent site of peroxyl radical attack with hydroperoxide formation in PP involves the tertiary centers, so that the chemiluminescence probably arises (1) from primary or methylperoxyl radicals formed from -scission,... 

Hamalainen, T.I., Sundberg, S., Makinen, M., Kaltia, S., Hase, T., and Hopia, A. (2001) Hydroperoxide Formation During Autoxidation of Conjugated Linoleic Acid Methyl Ester, Eur. J. Lipid Sci. Technol. 103, 588-593. 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Autoxidation of alkanes generally promotes the formation of alkyl hydroperoxides, but d4-tert-huty peroxide has been obtained in >30% yield by the bromine-catalyzed oxidation of isobutane (66). In the presence of iodine, styrene also has been oxidized to the corresponding peroxide (44). 

Figure 10.7 Autoxidation of a linoleic acid ester. In step 1 the reaction is initiated by the attack of a radical on one of the hydrogen atoms of the -CH2-group between the two double bonds this hydrogen abstraction produces a radical that is a resonance hybrid. In step 2 this radical reacts with oxygen in the first of two chain-propagating steps to produce an oxygen-containing radical, which in step 3 can abstract a hydrogen from another molecule of the linoleic ester (Lin-H). The result of this second chain-propagating step is the formation of a hydroperoxide and a radical (Lin ) that can bring about a repetition of step 2.

In plant tissues, various enzymes convert the hydroperoxides produced by LOX to other products, some of which are important as flavor compounds. These enzymes include hydroperoxide lyase, which catalyzes the formation of aldehydes and oxo acids hydroperoxide-dependent peroxygenase and epoxygenase, which catalyze the formation of epoxy and hydroxy fatty acids, and hydroperoxide isomerase, which catalyzes the formation of epoxyhydroxy fatty acids and trihydroxy fatty acids. LOX produces flavor volatiles similar to those produced during autoxidation, although the relative proportions of the products may vary widely, depending on the specificity of the enzyme and the reaction conditions. 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

The mechanisms responsible for inhibited oxidation depend on the experimental conditions and particular properties of RH and antioxidant (see earlier). Let us assume that hydroperoxide is relatively stable, so that it virtually does not decompose during the induction period (kdr -c 1). Actually, this means that the rate of ROOH formation is much higher than the rate of its decomposition, / 2[RH] [RO]2 ] 3> d[ROOH]. For each of the mechanisms of inhibited autoxidation, there is a relationship between the amounts of the inhibitor consumed and hydroperoxide produced (see Tablel4.2). For example, for mechanism V with key reactions (2), (7), (—7), and (8), we can get (by dividing the oxidation rate v into the rate of inhibitor consumption) the following equation ... 

In the case of diastereomeric mixtures of chiral hydroperoxides, standard chromatography on achiral phase can be employed to separate the diastereomers. As one example for the preparation of optically pure hydroperoxides via this method, the ex-chiral pool synthesis of the pinane hydroperoxides 11 is presented by Hamann and coworkers . From (15 )-cw-pinane [(15 )-cw-10], two optically active pinane-2-hydroperoxides cA-lla and trans-llb were obtained by autoxidation according to Scheme 17. Autoxidation of (IR)-c -pinane [(17 )-cw-10] led to the formation of the two enantiomers ent-lla and ent-llh. The ratio of cis to trans products was 4/1. The diastereomers could be separated by flash chromatography to give optically pure compounds. 

The dialkyl denomination also includes cyclic peroxides (endoperoxides). The most significant route for peroxide formation is probably that of autoxidation of organic materials, leading to their gradual degradation. Although hydroperoxides are the main products of this process, also peroxyesters are formed, as is the case, for example, of isoprostane bicyclic endoperoxides (25) mentioned in Section II.A.2.C. 

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