Autoxidation autocatalytic reactions

Upon exposure to air, animal and vegetable fats and oils become rancid (i.e., develop color changes and a musty, rank taste and odor). Here, the hydrogen atoms of the —CH2—groups located between alternating double bonds (i.e., —CH=CH—CH2—CH=CH—) of a polyunsaturated phospholipid or fatty acid (LH) are very susceptible to abstraction by free radicals. This process can then lead to a general reaction known as autoxidation, which results in the formation of a lipid hydroperoxide (LOOH) and the generation of a new free radical hence, an autocatalytic reaction results (lipid peroxidation). 

The behavior of 1-alkylpyrroles to autoxidation was studied by Smith and Jensen12 with 1-methyl-, 1-isopropyl-, and 1 -w-butylpyrrole. It was found that N-alkylpyrroles reacted much more slowly with oxygen than C-alkylpyrroles. The reactions were characterized by an induction period, during which the colorless liquid turned yellow and no oxygen uptake was detected successively an autocatalytic reaction took place. The simple oxidation products formed in the case of 1-methylpyrrole were isolated and the structures 10-13 assigned. 

The self-accelerating oxidation of hydrocarbons is called autoxidation. Its initial stage is characterised by a slow reaction with oxygen followed by a phase of increased conversion until the process comes to a standstill. The degradation is driven by an autocatalytic reaction described by the well-established free radical mechanism [1, 2], consisting of four distinct stages ... 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

In recent years Emanuel, Neiman, and their respective schools have greatly contributed to the theory of antioxidant action by studying the phenomenon of the critical antioxidant concentration in terms of a degenerate branched chain reaction. The critical antioxidant concentration, a well-established feature of phenolic antioxidants, is one below which autoxidation is autocatalytic and above which it proceeds at a slow and steady rate. Since the theory allowed not only a satisfactory explanation of the critical antioxidant concentration itself but elucidation of many refinements, such as the greater than expected activity of multifunctional phenolic antioxidants (21), we wondered whether catalyst-inhibitor conversion could be fitted into its framework. If degenerate chain branching is assumed to be the result of... 

AUTOXIDATION. A word used to describe those spontaneous oxidations, which take place with molecular oxygen or air at moderate temperatures (usually below 150°C) without visible combustion. Autoxidation may proceed through an ionic mechanism, although in most cases the reaction follows a free radical-induced chain mechanism. The reaction is usually autocatalytic and may be initiated thermally, photoehemically, or by addition of either free radical generators or metallic catalysts. Being a chain reaction, the rate of autoxidation may be greatly increased of decreased by traces of foreign material. 

Molecular Order. The autoxidation of linoleic acid in monolayers at 60°C differed from that in bulk in that it involved no lag periods, was considerably faster, and exhibited first order kinetics implying that the overall reaction is not autocatalytic (41). More recently... 

Food lipids possess an inherent stability to oxidation, which is influenced by the presence of antioxidants and pro-oxidants. After a period of relative stability (induction period), lipid oxidation becomes autocatalytic and rancidity develops. Thus, the typical time-course of autoxidation, as measured by the concentration of hydroperoxides, consists of a lag phase (induction) followed by the rapid accumulation of hydroperoxides, which reaches a maximum and then decreases as hydroperoxide decomposition reactions become more important. The longer the induction period, the more stable the food to oxidation (Lundberg, 1962). 

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

Other typical chain reactions include those of hydrogen with halogen in the gas phase and oxidation of organic substances at moderate temperatures (autoxidation). A special facet of the latter reactions is that the product or an intermediate can act as initiator, and the reaction then is autocatalytic. 

The chain reaction is initiated by abstraction of an allylic hydrogen to give an allylic radical stabilized by delocalization over three or more carbons. The initiator is a free radical, most probably produced by decomposition of hydroperoxides already present or produced by photooxidation. The decomposition may be thermal, but it is more likely promoted by traces of variable redox state metal ions. Autoxidation is characterized by an induction period during which the concentration of free radicals increases until the autocatalytic propagation steps become dominant. During the induction period, there is little increase in oxidation products. 

Mechanistic studies of autoxidation have concentrated on methylene-interrupted fatty acids, but many of the observations are valid for other compounds. Conjugated fatty acids such as CLA also oxidize through an autocatalytic free radical reaction, with the predominant hydroperoxide determined by the geometry of the conjugated diene system (45). Other groups with activated methylenes may be susceptible to oxidation, for example, the ether methylenes of ethoxylated alcohols used as surfactants (46). 

Molecular oxygen is the major cause of irreversible deterioration of hydrocarbon substrates, leading to the loss of useful properties and to the ultimate failure of the substrate. The oxidation process of hydrocarbons is autocatalytic oxidation starts slowly, sometimes with a short induction period, followed by a gradual increase in the rate, concomitant with the build up of hydroperoxides, which eventually subside, giving rise to a sigmoidal oxidation curve. When initiators such as peroxides are present, the length of the induction period is absent, or very short, but it can be prolonged by antioxidants, as shown in Fig. 1. The basic autoxidation theory of hydrocarbons involves a complex set of elementary reaction steps in a free radical-initiated chain reaction mechanism the basic tenets of this theory apply equally to polymer oxidation. 

Earlier Morgan (1967) showed that the autoxidation of Mn(II) in water was an autocatalytic process in that the solid products of the reaction were shown to accelerate the rate of reaction, which was described by the following rate law ... 

The overall rate increases autocatalytically until a limiting rate is reached. At this point the concentration of hydroperoxide available for initiation reaches a constant value. In many cases the steady state concentration of hydroperoxide is low, autoxidation rates are high and kinetic chain lengths short. Most of the hydrocarbon is consumed by reactions with other than peroxy radicals . 

The radical chain mechanism outlined here avoids the ineffective direct reaction of molecular oxygen with the substrate hydrocarbon. The fast propagation reactions produce ROOH that in turn can initiate new radical chains. As the primary product of the reaction initiates new reactions, one ends up with an autocatalytic acceleration. The propagating peroxyl radicals can also mutually terminate and yield one molecule of alcohol and ketone in a one-to-one stoichiometry. The ratio between the rate of propagation and the rate of termination is referred to as the chain length and is of the order of 50-1000. As the desired chain products are more susceptible to oxidation, autoxidations are normally carried out at low conversions in order to keep the selectivity to an economically acceptable level. 

Autoxidation is the process in foods and bulk lipids, which leads to rancidity. Rancidity is the spoiled off-flavor obtained by subjective organoleptic appraisal of food. Autoxidation is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical mechanism. This indicates that the intermediates are radicals (odd electron species) and that the reaction involves an initiation step and a propagation sequence, which continues until the operation of one or more termination steps. Autoxidation of lipid molecules is briefly described by reactions 1-3. ... 

Very frequently, the phenomena described in the previous section are observed qualitatively but with considerably less sharpness, in the kind of autocatalysis associated with degenerate chain branching. Here, the active center involved in the chain branching step is not an active center at all but a relatively unstable intermediate product which, upon its decomposition or reaction provides active centers at a rate considerably faster than that of the original initiation. Thus the autocatalytic behavior can really be ascribed to a secondary initiation brought about by an intermediate product. This phenomenon happens frequently in the oxidation of hydrocarbons RH. At low temperatures, it is called autoxidation and it is autocatalytic because of the further decomposition into free radicals of hydroperoxides ROOH which are first produced in the oxidation (see p. 101). 

The autoxidation of unsaturated fatty acids is a chain process occurring autocatalytically through free radical intermediates. Although autoxidation implies that this reaction occurs spontaneously under mild conditions, it is generally initiated by trace metals and peroxides or hydroperoxides present as ubiquitous impurities in food and biological lipid systems (see Chapter 1). 

These break down to give shorter-chain products, including free radicals, which then attack other fatty adds much more readily than does the original oxygen. More free radicals are produced, with the result that the speed of the oxidation increases exponentially. Eventually the concentration of free radicals becomes such that they react with each other and the reaction is terminated. Such a reaction, in which the products catalyse the reaction, is described as autocatalytic. This particular reaction is an autoxidation. The formation of the free radicals is catalysed by ultraviolet hght and certain metal ions, particularly copper, and the presence of either increases the rate of oxidation dramatically. 

Inhibited autoxidation is often characterized by critical phenomena reasoned by the autocatalytic character of the reaction and mentioned above feedback. Since hydroperoxide decomposes during oxidation, two different regime of inhibited oxidation appear non-stationary and quasi-stationary with respect to hydroperoxide. 

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