Autoxidation propagation reactions

Tables I and II include data for the co-oxidations of styrene and butadiene in chlorobenzene and ferf-butylbenzene solutions, as well as with no added solvent. These solvents were chosen because the rate of oxidation of cyclohexene varies significantly in them at the the same rate of initiation (6). There is a variation in the over-all rate of oxidation under these solvent conditions, but there appears to be no significant difference in the measured ra and rb (Table II). If the solvent does affect the propagation reaction in autoxidation reactions, it affects the competing steps to the same degree.

A specific free radical can be produced from a precursor molecule either in an initiation step or a propagation step in which a reagent radical reacts with the precursor. Initiation requires either removal or addition of an electron or homolysis. Chemically this can be done in a number of ways, by using one-electron oxidants or reductants or by inducing homolysis in some way examples of these types of reactions include autoxidation [84-86], photochemical oxidation and reduction [87-90], and oxidation and reduction by metal ions and their complexes [91-93], In propagation reactions, the reagent radical might be the hydroxyl radical, the hydrated electron, or any other suitably reactive species that will interact with the precursor molecule in the desired manner. We will consider initiation reactions first. 

The cobalt-catalyzed autoxidation of toluene in acetic acid at 363 K is accelerated by butan-2-one and benzaldehyde because peroxy radicals play a minor role in ratecontrolling propagation reactions. High rates of autoxidation are also obtained in the presence of Br because bromine atoms are important chain-propagating species. ... 

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. 

Reaction [215] represents the initial event in thiyl radical formation as proposed for the so-called spontaneous autoxidation of thiols catalysed by metal ion complexes (Misra 1974, Saez et al. 1982, Harman et al. 1984). The propagation (reactions [216] and [217]) leads to thiol oxidation and contemporary superoxide radical formation. The latter might be reduced to hydrogen peroxide by reacting with (EDTA)2-Mn " to yield back the oxidised complex and close the catalytic cycle. 

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). 

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. 

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.

The kinetic analysis proves that formation of very active radical from intermediate product can increase the reaction rate not more than twice. However, the formation of inactive radical can principally stop the chain reaction [77], Besides the rate, the change of composition of chain propagating radicals can influence the rate of formation and decay of intermediates in the oxidized hydrocarbon. In its turn, the concentrations of intermediates (alcohols, ketones, aldehydes, etc.) influence autoinitiation and the rate of autoxidation of the hydrocarbon (see Chapter 4). 

This problem was first approached in the work of Denisov [59] dealing with the autoxidation of hydrocarbon in the presence of an inhibitor, which was able to break chains in reactions with peroxyl radicals, while the radicals produced failed to contribute to chain propagation (see Chapter 5). The kinetics of inhibitor consumption and hydroperoxide accumulation were elucidated by a computer-aided numerical solution of a set of differential equations. In full agreement with the experiment, the induction period increased with the efficiency of the inhibitor characterized by the ratio of rate constants [59], An initiated inhibited reaction (vi = vi0 = const.) transforms into the autoinitiated chain reaction (vi = vio + k3[ROOH] > vi0) if the following condition is satisfied. 

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. 

This difficulty has now been overcome. Howard, Schwalm, and Ingold (24) show that the rate constant for reaction of any alkylperoxy radical with any hydrocarbon can be determined (by the sector method) by carrying out the autoxidation of the hydrocarbon in the presence of >0.1 M hydroperoxide corresponding to the chosen radical. All the absolute propagation and termination constants for the co-oxidation of cumene and Tetralin were thus determined. Our Tetralin-cumene work suggests that their results agree well with the best we have been able to get... 

In the early stages of the autoxidation of chloroprene the amount of oxygen absorbed increased as the square of the time. This dependence on time is frequently observed in autoxidations and is an approximation to that expected for an oxidation of long chain length, initiated by the first-order decomposition of the peroxidic product and terminated by a bimolecular reaction of the propagating peroxy radicals. 

The hydroperoxide radical reacts with another molecule of oxygen (Reaction 5) to give the hydroperoxide-peroxy radical. This radical in turn reacts with a molecule of dihydroanthracene (Reaction 6), to give the dihydroperoxide and generate a radical to propagate the chain. However, the hydroperoxide radical formed in Reaction 4 may be decomposed by a carbanion to the anthracene diradical (Reaction 7). [An example of the decomposition of an unstable hydroperoxide by reaction with an anion is found in the basic autoxidation of 2-nitropropane (3).]... 

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