Autoxidation termination reactions

It should be clear from Section IV. B that a major difficulty involved in preparing monomeric iron-dioxygen adducts is the prevention of bimolecular termination reactions, leading via autoxidation to the formation of a ju-oxo dimer, thus... 

Where B = pyridine, piperidine or 1-methylimidazole, in methylene chloride solution, but under normal conditions rapid irreversible autoxidation takes place 232) leading to the formation of the well characterised 247, 248) fi-oxo product, (TPP)Fe(IlI)—0—Fe(III) (TPP) and since the rate of oxidation decreases 249, 250) with increasing excess of axial base, B, it follows 232, 251) that a five co-ordinate species, Fe(II) (Base)TPP, is probably involved as an intermediate which can then undergo a bimolecular termination reaction with Fe(II) (Base)02TPP, followed by autoxidation. Firstly 251),... 

Howard and Ingold (7) proposed participation of a first-order termination reaction in the autoxidation of styrene. If such contributions from first-order terminations are real and widespread in autoxidation, more knowledge about them becomes essential. 

Hendry and Russell (15) have shown how polyarylmethanes and polyarylethylenes may retard autoxidations of other hydrocarbons at 60° to 90 °C. because the carbon radicals involved react relatively slowly and incompletely with oxygen, the free arylmethyl radicals contribute to a fast, crossed, termination reaction. A similar effect has been reported for benzyl radicals above 300°C. (11). These effects can usually be overcome by sufficient oxygen pressure hence, our restriction at the beginning of this section. 

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,... 

Degenerate chain branching may occur between various radicals produced in the autoxidation sequence, and involves bi-radical termination reactions. 

Fig. 3. Autoxidation of polyunsaturated fatty acids in phospholipid membranes. Addition of oxygen to lipid free radicals is extremely fast. It yields peroxyl radicals ROO which will tend to capture labile hydrogen atoms of neighbouring polyunsaturated lipids. Accidentally produced free radicals will therefore initiate a chain reaction of lipid peroxidation which will propagate along membranes. This process can result in several dozen propagation steps before it is stopped by a termination reaction. Examples of such termination reactions are the recombination of peroxyl radicals and the formation of a stable free radical from a free radical scavenger (scavH). Termination through recombination of low steady-state concentration of alkyl radicals is unlikely in aerobic medium.

Kinetic, isotopic and product studies of autoxidation suggest that these reactions and reaction (42) proceed via a tetroxide intermediate (RO4R) which may be a transition state or a molecule of finite lifetime [151]. Russell [152] has suggested that the termination reactions of primary and secondary alkylperoxy radicals in fact involve the formation of a cyclic transition state, viz. 

Autoxidation can be inhibited or retarded by adding low concentrations of chainbreaking antioxidants that interfere with either chain propagation or initiation (286). Chain-breaking antioxidants include phenolic and aromatic compounds hindered with bulky alkyl substituents. Common synthetic chain-breaking antioxidants used in food lipids include butylated hydroxyanisole (BHA), butylated hydroxyto-luene (BHT), ferf-butyUiydroquinone (TBHQ), and propyl gallate (PG). This class of antioxidants react with peroxy free radicals to terminate reaction chains. The antioxidant radical (A ) formed in Equation 5 should be relatively stable and unable to initiate or propagate the oxidation chain reaction. 

Termination of the radical chain reaction As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. This autotermination is called the chain termination reaction and dominates in this final phase of the oxidation process such that degradation comes to a halt. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols. Reaction sequence (4.14) ... 

We can estimate an upper limit for the autoxidation as follows. We assume that despite the fact that we do not observe any catalyzed CHP decomposition even after 120 hours at 65°C, we still decompose it at a rate of 1% over a period of 120 hours. Using this rate for radical production, the normal solution rate constants for the propagation and termination reactions and the formulae given in Ref. 10, we estimate the rate of CHP production to be 0.54 X 10 Ms The observed rate, 2.6 x 10 Ms l, is thus about 5 times larger then the upper limit of the estimated initiated free-radical rate. 

The reaction of alkanes with a mixture of SO2 and O2 also occurs more readily than simple autoxidation of the hydrocarbon (reaction 30). Here the radical RSO2OO , formed as shown in Scheme 27, is apparently less prone to termination reactions than a simple alkylperoxy radicaF. ... 

A review of autoxidation and autoxidation kinetics has been published. A DFT study of autoxidation of diethyl ether (DEE) supported the basic mechanism involving steps such as chain initiation, propagation, and termination reactions as in alkane oxidations but inferred that the reaction could be different in the presence or absence of... 

At the last stages of autoxidation the peroxyl radicals accumulate. At relatively high levels the peroxyl radicals interact with each other to form non-radical products by the termination reaction (11),... 

The hydroperoxide method involves autoxidation of a substrate, RH, in the presence of enough hydroperoxide of another substrate, R OOH, so that rate controlling propagation and termination reactions only involve peroxyls derived from the hydroperoxide. 

By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers. Reaction 20 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of alkylperoxy radicals. The resulting PINO radical subsequently abstracts a hydrogen atom from the a-C-H bond of the alcohol to propagate the autoxidation chain (Reactions 21-23). 

If the concentration of free radicals in the reaction system is quite high, it is likely that two free radicals react together to form a relatively stable product, and the chain reaction ends. This third stage is called the termination stage (termination) of an autoxidation reaction. With a limited supply of oxygen, when the rate of autoxidation depends on its partial pressure, the main radicals in the system are fatty acid radicals (R ) and the main termination reaction is their recombination. With an adequate oxygen supply, the reaction rate is independent of its partial pressure. This creates higher amounts of peroxyl radicals (ROO ) and the main termination reactions are then recombination of radicals of the fatty adds (R ) with peroxyl radicals and mutual recombination of peroxyl radicals. 

Since the effects of heavy metals increase the amount of free radicals in the lipid phase, not only do the rates of initiation and propagation reactions increase, but also the rate of termination reaction increases. Heavy metals therefore also change the composition of the reaction products. At high concentrations of free radicals, the termination reaction may dominate and metals then act as the inhibitors of autoxidation. Autoxidation reaction can also be inhibited by metals when they are present at higher concentrations. It is assumed that the reason is the oxidation and reduction of free hydrocarbon radicals to anions and cations by ions of Fe and Cu and the formation of complexes of free radicals. Other complexes are also formed with Co. All these reactions interrupt the radical chain autoxidation reaction. Reactions with Fe ions are given as examples. 

Antioxidants (see Section 11.2.2) are substances that can react with free radicals of the autoxidation chain, especially with peroxyl radicals (Figure 3.66). The reaction creates hydroperoxides or other non-radical Hpid products. The antioxidant is transformed to the form of a free radical, which, however, is fairly stable, so it is unable to continue in the autoxidation reaction. The role of the antioxidant thus lies in shortening the autoxidation chain and increasing the rate of termination reactions. During the reaction the antioxidant is consumed. When aU of the antioxidant has been consumed, the autoxidation reaction proceeds as if no antioxidant was present. Antioxidants therefore cannot completely stop the autoxidation reaction they just slow this reaction down, ideally to the initial reaction rate. 

Prior to 1957 the termination reaction for liquid-phase hydrocarbon autoxidation at oxygen pressures above oa. 100 torr was generally written as... 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

Product analyses (ll, 31, ) have shown that the cmylperoxy radical undergoes non-terminating and terminating reactions during autoxidation of cijmene. 

Fiikuzumi and Cuo have very recently concluded that the termination reaction for oxidation of c miene with manganese dioxide or cobalt oxide supported on silica ( ) and during autoxidation of cumene initiated by reaction of cumene hydroperoxide with lead oxide ( ) is strictly first-order with respect to the concentration of cumylperoxy radicals. These workers proposed an unprecedented 1,3-methyl shift followed by 0-0 bond cleavage to account for these inusual kinetics. 

In many polymers luminescence above the glass transition temperature due to increased molecular motion increases sample light emission. However, some polymers behave exactly the opposite with decreased light intensity in samples exposed to temperatures above the glass transition. The rate determining step is the termination reaction of free radicals. For Nylon 6/6, increases in sample temperature increase chemiluminescence due to quenching of the excited carbonyl C=0. A model of the mechanism of chemiluminescence applied to Nylon 6,6 is reported in the Appendix. Based on this model, oxygen is never consumed in an autoxidation reaction. 

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

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