Autoxidation chain reaction

The behavior of the 4-nitro derivative is surprising since the anti-oxidative efficiency is known to be adversely affected by the presence of electronegative groups (21), limiting the participation of antioxidants in breaking the autoxidation chain reaction, as shown in Reaction 1. 

Antioxidants, such as 2,6-di-te/ /-butyl-4-mcthylphenol (also known as butylated hydroxytoluene or BHT) and 2-/ert-butyl-4-methoxyphenol (also known as butylated hydroxyanisole or BHA), are added to many organic materials to prevent autoxidation. They function by interfering with the autoxidation chain reaction. When a radical encounters an antioxidant molecule, such as BHA, it abstracts a hydrogen to produce a resonance-stabilized radical ... 

Polymers are capable of absorbing atmospheric oxygen, which may then induce autoxidative chain reactions in the polymer, resulting in various low-molecular alcohols, ketones, aldehydes and carboxylic acids. 

A short review of photo-oxidation of a range of polymers by Faucitano and co-workers [30] summarised the understanding at the time of PET photolysis as a splitting of C-0 bonds in the ester gronps, with formation of acyl and carboxyl radicals, which themselves can lose carbon oxides to produce phenyl or alkyl radicals, or abstract hydrogen to produce aldehydes and carboxylic acids. When oxygen is present, the authors state that the autoxidation chain reaction will lead to formation of anhydrides and aldehydes, and to hydroxy-substituted phenyl species. 

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). 

With an adequate supply of oxygen, the hydrocarbon radicals (R ), formed in the initial phase of the autoxidation chain reaction, preferentially yield hydroperoxyl radicals (ROO ). When there is a limited supply of oxygen (with a low partial pressure of oxygen) and in the absence of antioxidants, hydrocarbon radicals react with each other and form lipid dimers (R R). In the presence of tocopherols, there is a competitive reaction of the hydrocarbon radicals with tocopheroxyl radicals that are stabilised by the formation of hydrocarbons (R H) and other stable products ... 

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). 

The mechanism by which an oiganic material (RH) undergoes autoxidation involves a free-radical chain reaction (3—5) ... 

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

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. 

The situation is different when I autoxidation processes belonging to the category of induced chain reactions. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

Dichlorine shortens the induction period of autoxidation of paraffin wax [187] and accelerates the oxidation of hydrocarbons [109]. Difluorine is known as very active initiator of gas-phase chain reactions, for example, chlorination [188,189]. 

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). 

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] ... 

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. 

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). 

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. 

Formulas for Kinetic Parameters of Hydrocarbon Autoxidation as Chain Reaction in Quasistationary Regime Chain Length v, Critical Concentration of Inhibitor [lnH]cr, and Quasistationary Concentration of Hydroperoxide [ROOH]s. The Following Symbols are Used / = k3/ka and V[0 is the Rate of Free Radical Generation on Reaction of RH with Dioxygen [33,38,45]... 

Catalysis by radicals will usually be due to a radical addition or displacement reaction, hydrogen and halogen being the atoms on which the displacement most often occurs. It is usually a chain reaction once the substrate is converted into a radical it carries the reaction to many molecules of substrate. Examples are polymerization and autoxidation. 

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. 

Note Autoxidation is usually the result of chain-reaction with air or oxygen, and the intermediate products are usually peroxidic in nature. 

Autoxidation of mercaptans gives rise to thiyl radicals, but these, like phenoxy radicals, are inert toward oxygen and normally dimerize to disulfides. Their participation in a chain reaction can be achieved in the co-oxidation of olefins and mercaptans, first demonstrated by Khar-asch (12), which takes advantage of the rapid addition of thiyl radicals to double bonds. 

AIBN is, however, virtually unaffected by the presence of dithio-phosphates (Table II). Further, with specific reference to the oxidation of the disulfide in Table I, which has no effect on the rate of AIBN-initi-ated autoxidation of cumene (6), it is unlikely that the efficiency of radical production from AIBN increases since this would produce a prooxidant effect in cumene. Thus, the zinc salt inhibitor is being oxidized in competition with the main chain reaction. 

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