Inhibited autoxidation

Inhibit autoxidation of organic materials by interfering with free radical reactions that lead to incorporation of oxygen into macromolecules in a chain mechanism consisting of two interacting cyclical processes (Scheme II.1). 

As already noted (see Chapter 4), autoxidation is a degenerate branching chain reaction with a positive feedback via hydroperoxide the oxidation of RH produces ROOH that acts as an initiator of oxidation. The characteristic features of inhibited autoxidation, which are primarily due to this feedback, are the following [18,21,23,26,31-33] ... 

As shown above (see earlier) for straight chain reactions, the inhibitor is consumed at a constant rate v-Jf Similarly, during the inhibited autoxidation of RH, the inhibitor is initially consumed at a constant rate vi0/f, but then the rate of inhibitor consumption drastically increases [57,58], which leads to a rapid accumulation of hydroperoxide and the enhancement of initiation (see Figure 14.1). 

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 ... 

Autoxidation (interaction of a substance with molecular oxygen at below 120°C without flame [1]) has often been involved in the generation of hazardous materials from reactive compounds exposed to air. Methods of inhibiting autoxidation of organic compounds in the liquid phase has been reviewed [2,3],... 

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

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. 

Quasistationary Kinetics of Inhibited Autoxidation Topological Kinetics of Inhibited Oxidation of Hydrocarbons... 

We can now calculate the concentration of the peroxyl radicals in this inhibited autoxidation. The principal termination reaction is now reaction with vitamin E, rather than reaction 8a as had been previously true. Therefore, we can write Equations 19 and 20,... 

A topic related to the foregoing discussion concerns metal complexes, such as many sulfur-containing metal chelates,357 364 that are capable only of inhibiting autoxidations. The detailed mechanisms of the inhibiting action of these metal complexes are not very well understood. Recent results362 suggest that zinc dialkyldithiophosphates react with alkylperoxy radicals at the metal center, which could involve electron transfer or an SH2 reaction ... 

The most common dioxygen carriers of nature, the heme proteins hemoglobin and myoglobin, bind Oj in a characteristic structure. The Oj molecule binds to the metal atom through one of its atoms, and the M—O—O unit is angular in shape. Studies on small molecules that bind O2 in this way have concentrated on two classes of ligands, porphyrins and Schiff bases.In the first case, bulky superstructures are usually appended to the porphyrin ligand this superstructure inhibits autoxidation of the metal atom in the... 

This simple kinetic treatment of inhibited autoxidation provides for a useful semiquan-titative explanation of what is meant by antioxidant and antioxidant activity under known and controlled Rj. The ability of a known amount of potential antioxidant to suppress the oxygen uptake depends on the value of the absolute rate constant for inhibition,... 

Organometallic complexes of transition metals Transition metals are characterised by their ability to exist in more than one oxidation state, which gives them catalytic activity in many chemical and biological processes. For the autoxidation and stabilisation of hydrocarbons such as mineral oil, it is proposed that transition metals can either catalyze or inhibit autoxidation by the following set of reactions [37]. In Reaction sequences (4.38) and (4.39), transition metals can catalyze... 

Phospholipid content is normally calculated from the determination of total phosphorus and the use of a factor relating the molecular weight of phosphorus to the mean molecular weight of the phospholipids in the oil. Typically, the amount of phospholipids in cottonseed crude oil varies from about 0.7-0.9%. The phospholipids can also be beneficial in that they act as synergists for the tocopherols which inhibit autoxidation of vegetable oils. This synergistic effect is partly responsible for the oxidative stability of crude cottonseed oil (Jones and King 1996). 

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