Inhibition of autoxidation

Trapping of radicals by scavengers belongs to the most important methods to prevent autoxidation reactions. Radical scavengers are also referred to as chain breaking acceptors. Further, they are classified as primary antioxidants. 

Hydrogen donors are referred to as chain breaking donors. In particular, hydrogen donors decompose peroxides into inert products. Hydrogen donors are classified as primary antioxidants, because they access secondary products in the chain of the autoxidation reaction. The mechanism that interferes the autoxidation reaction is shown in Eq. 19.5. 

Hydroperoxides are still reactive in another manner as they may form peroxide radicals. Hydroperoxide decomposers react with 

Thio compounds are active in the same way. Eq. 19.7 shows the mechanism of peroxide decomposition using thio compounds. 

Phosphite and phosphonite esters act as antioxidants by three basic mechanisms depending on their structure (1). Basically, phosphites and phosphonites are secondary antioxidants that decompose hydroperoxides. Their performance in hydroperoxide decomposition decreases from phosphonites, alkyl phosphites, aryl phosphites, down to hindered aryl phosphites. Five membered cyclic phosphites act catalytically by the formation of acidic hydrogen phosphates. In contrast, hindered aryl phosphites are interrupting the branched kinetic chain. 

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The accumulation of hydroxyl-containing products, such as hydroperoxides, alcohols, acids, and water, also reduce the total activity of peroxyl radicals due to the hydrogen bonding with R02 [150], When acting together, these factors cause self-inhibition of autoxidation at conversion levels of 40-50% [3],... 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

Inhibition of autoxidations by transition metals in low oxidation states, such as Co(II) or Mn(II), has often been observed.18 142 143,356 Transition metal complexes often behave as catalysts at low concentrations but as inhibitors at high concentrations.142,143 There has been some question as to the cause of this phenomenon. 

Contrary to our results, other workers (4, 9, 20, 36) state that in the stabilization of carotene, paraffin wax, and lard the activity of pyrocatechol is favorably affected by substitution at position 4, not only by normal but by tertiary alkyl groups as well. Disparate influences of substitution are not surprising when comparing the activity in different substrates owing to the possibility of directive influences in the process of inhibited oxidation. The participation of phenolic antioxidants in the inhibition of autoxidation can be demonstrated (1, 2, 3) simply as a reaction between the molecule of antioxidant AH and the alkylperoxy radical ROO formed duririg the autoxidation of the substrate RH. During this process, an aryloxy radical (A ) is first generated. 

In 1924 in a note on negative catalysis (12a) Christiansen s views were further substantiated. This paper induced Backstrom to enter on his well-known investigations on inhibition of autoxidations (18). 

Several workers have observed the inhibition of autoxidative degradation of cellulose or its model compounds in the presence of magnesium compounds (8-14, 18). However, the mechanism for the observed inhibition process has been a matter of some controversy. Some workers have attributed their observation to the stabilization of peroxides by magnesium compounds (8, 9, 11). Robert has suggested that the metallic catalysts are adsorbed on or coprecipitated with magnesium hydroxide (13). Other workers are convinced that the... 

Figure 4. Inhibition of autoxidation of styrene by N-oxyls and hydroxylamines.

Although inhibition of autoxidation by donation of hydrogen to peroxy radicals, reaction (5), is an important reaction, Boozer and Hammond [7] have suggested that inhibition by complex formation may also be important. Assuming that the major termination step involves reactions (9) and (10), and that the reaction is initiated by azobisisobutyro-nitrile (AIBN), then the rate of initiation is... 

In the following years, nanostructural differences in the particle morphology and thus differing gas diffusivity was identified as the cause of a different oxidative stability of microencapsulated nutritional oils. When using a octenylsuccinate-derivatized starch with a high proportion of low molecular weight disaccharides, a significant inhibition of autoxidation compared to a carrier... 

As was noted by Jones (ref. 12) the success of a metal bromide as a catalyst for alkylaromatic autoxidations depends on the ability of the metal to transfer rapidly and efficiently oxidizing power from various autoxidation intermediates onto bromide ion in a manner which generates Br-. The fact that no free bromine is observable in this system is consistent with rapid reaction of intermediate bromine atoms with the substrate. Inhibition of the reaction by cupric salts can be explained by the rapid removal of Br2 or ArCH2- via one-electron oxidation by Cu (Fig. 10). 

In model studies involving Fe(n) species, three broad approaches have been used to mitigate the problem of autoxidation of the iron (Hay, 1984). These are (i) the use of low temperatures so that the rate of oxidation becomes very slow (ii) the synthesis of ligands containing steric barriers such that dimerization of the iron complex is inhibited, and (iii) immobilization of the iron complex on a solid surface such that dimerization once again will not be possible. 

Kinetics of Autoxidation of Organic Compounds Inhibited by Acceptors of Peroxyl Radicals... 

KINETICS OF AUTOXIDATION OF ORGANIC COMPOUNDS INHIBITED BY ACCEPTORS OF PEROXYL RADICALS... 

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. 

For initiated oxidation, the inhibitory criterion could be defined as the ratio v0/v or (v0/ v — v/v0), where v0 and v are the rates of initiated oxidation in the absence and presence of the fixed concentration of an inhibitor, respectively. Another criterion could be defined as the ratio of the inhibition coefficient of the combined action of a few antioxidants / to the sum of the inhibition coefficients of individual antioxidants when the conditions of oxidation are fixed (fx = IfiXi where f, and x, are the inhibition coefficient and molar fraction of z th antioxidant terminating the chain). It should, however, be noted that synergism during initiated oxidation seldom takes place and is typical of autoxidation, where the main source of radicals is formed hydroperoxide. It is virtually impossible to measure the initial rate in the presence of inhibitors in such experiments. Hence, inhibitory effects of individual inhibitors and their mixtures are usually evaluated from the duration of retardation (induction period), which equals the span of time elapsed from the onset of experiment to the moment of consumption of a certain amount of oxygen or attainment of a certain, well-measurable rate of oxidation. Then three aforementioned cases of autoxidation response to inhibitors can be described by the following inequalities (r is the induction period of a mixture of antioxidants). 

The equations for the rate of oxidation of organic compound RH inhibited by a mixture of InH and Q and the characteristics of autoxidation when the mechanism includes different combinations of key reactions are presented in Table 18.8. 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

The present paper reports the results of a kinetic study of the inhibition of the azobisisobutyronitrile-initiated autoxidation of cumene at 60 °C. and of Tetralin at 70 °C. by zinc diisopropyl dithiophosphate, undertaken to test the validity of the chain-breaking inhibition mechanism proposed above. In addition, the effectiveness of several metal dialkyl dithiophosphates as antioxidants in the autoxidation of squalane... 

Chain-Breaking Inhibition Mechanism. According to the mechanism proposed earlier (6), the inhibition of the autoxidation of a hydrocarbon... 

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. 

For experimental reasons (discussed below) the relative rate of decrease should be smaller than 0.01 sec."1. Since kA is of the order of 104 liter per mole per sec., [AH] must be of the order of 10 6 mole per liter. This low concentration of inhibitor will only have an appreciable effect on the rate of autoxidation if the rate of initiation—and consequently the rate of oxidation—is low. To measure the decrease in rate during the short-lived non-steady state, one must be able to determine these low velocities within short periods of time. From the usual inhibition formulas one can compute, for instance, that in order to obtain a ratio of original to inhibited rate of about 5 with [AH] = 10 6 mole per... 

Human ceruloplasmin inhibits lipid autoxidation induced by ascorbate or inorganic Fe It is considered an acute-phase protein with a beneficial effect in inflammation . It was suggested that ceruloplasmin acts as a scavenger of OJ radicals, as it inhibited the reduction of Fe(III)-cytochrome c and of nitroblue tetrazolium in the presence of xanthine oxidase, acetaldehyde, and dioxygen as an OJ-generating system A mechanism without reduction of Cu , similar to that... 

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