Chain initiation, oxidation

Initiation. Free-radical initiators are produced by several processes. The high temperatures and shearing stresses required for compounding, extmsion, and molding of polymeric materials can produce alkyl radicals by homolytic chain cleavage. Oxidatively sensitive substrates can react directly with oxygen, particularly at elevated temperatures, to yield radicals. 

In the absence of added radical generators the relatively unstable hydroperoxides, which are themselves generated by the oxidation process, are the major source of chain initiating radicals. 

The fimction of an antioxidant is to divert the peroxy radicals and thus prevent a chain process. Other antioxidants fimction by reacting with potential initiators and thus retard oxidative degradation by preventing the initiation of autoxidation chains. The hydroperoxides generated by autoxidation are themselves potential chain initiators, and autoxidations therefore have the potential of being autocatalytic. Certain antioxidants fimction by reducing such hydroperoxides and thereby preventing their accumulation. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

However, peroxidation can also occur in extracellular lipid transport proteins, such as low-density lipoprotein (LDL), that are protected from oxidation only by antioxidants present in the lipoprotein itself or the exttacellular environment of the artery wall. It appeats that these antioxidants are not always adequate to protect LDL from oxidation in vivo, and extensive lipid peroxidation can occur in the artery wall and contribute to the pathogenesis of atherosclerosis (Palinski et al., 1989 Ester-bauer et al., 1990, 1993 Yla-Herttuala et al., 1990 Salonen et al., 1992). Once initiation occurs the formation of the peroxyl radical results in a chain reaction, which, in effect, greatly amplifies the severity of the initial oxidative insult. In this situation it is likely that the peroxidation reaction can proceed unchecked resulting in the formation of toxic lipid decomposition products such as aldehydes and the F2 isoprostanes (Esterbauer et al., 1991 Morrow et al., 1990). In support of this hypothesis, cytotoxic aldehydes such as 4-... 

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

Attempts to achieve selective oxidations of hydrocarbons or other compounds when the desired site of attack is remote from an activating functional group are faced with several difficulties. With powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. When a hydrocarbon is oxidized, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive oxidation of alcohol and carbonyl intermediates. There are a few circumstances under which oxidations of hydrocarbons can be synthetically useful processes. One group involves catalytic industrial processes. Much effort has been expended on the development of selective catalytic oxidation processes and several have economic importance. We focus on several reactions that are used on a laboratory scale. 

The border conditions between chain and nonchain mechanisms of oxidation depends not only on temperature but also on the hydrocarbon concentration and the rate of chain initiation. The following equation describes this dependence ... 

The decay of the biradical produces ketone molecule in the triplet state, which is an emitter of light [222], The CL intensity was proved to be propotional to the rate of chain initiation, which is equal to the rate of chain termination. The observed luminescence spectra were found to be identical with the spectra of the subsequent ketone in the triplet state. The intensity of CL (/chi) produced by oxidized hydrocarbon is the following ... 

Due to the high initiation rate and low (room) temperature, chains for oxidation of alkanes are short and many products are formed by disproportionation of peroxyl and hydroperoxyl radicals. The G values of the products of radiolytic oxidation of four alkanes are given in the following table [233] ... 

Scheme A. This scheme is typical of the hydrocarbons, which are oxidized with the production of secondary hydroperoxides (nonbranched paraffins, cycloparaffins, alkylaro-matic hydrocarbons of the PhCH2R type) [3,146]. Hydroperoxide initiates free radicals by the reaction with RH and is decomposed by reactions with peroxyl and alkoxyl radicals. The rate of initiation by the reaction of hydrocarbon with dioxygen is negligible. Chains are terminated by the reaction of two peroxyl radicals. The rates of chain initiation by the reactions of hydroperoxide with other products are very low (for simplicity). The rate of hydroperoxide accumulation during hydrocarbon oxidation should be equal to ...

Let us compare the ratio of radicals in oxidized 2-propanol and cyclohexanol at different temperatures when oxidation occurs with long chains and chain initiation and termination do not influence the stationary state concentration of radicals. The values of the rate constants of the reactions of peroxyl radicals (kp) with alcohol and decomposition of the alkylhydroxy-peroxyl radical (k ) are taken from Table 7.4 and Table 7.5. 

In thermal oxidation, chain initiation takes place by the reaction of the aldehyde with dioxygen. Two reactions of chain generation in autoxidized aldehydes, namely, bimolecular and trimolecular, were proved [25]. 

Amides, as amines and hydrocarbons, are oxidized by dioxygen according to the chain mechanism [1], The initiated oxidation of amides proceeds according to the classical scheme... 

The important characteristics of polymers oxidation were obtained as a result of the study of their initiated oxidation. In the presence of initiator (I) which generates the chains with the rate v, = /c,[I], the oxidation of polymer PH occurs with the constant rate v. When the macroradical P of the oxidized polymer reacts with dioxygen very rapidly (at [02]... 

Depending on the oxidation conditions and its reactivity, the inhibitor InH and the formed radical In can participate in various reactions determining particular mechanisms of inhibited oxidation. Of the various mechanisms, one can distinguish 13 basic mechanisms, each of which is characterized by a minimal set of elementary steps and kinetic parameters [38,43 15], These mechanisms are described for the case of initiated chain oxidation when the initiation rate v = const, autoinitiation rate fc3[ROOH] -C vy and the concentration of dissolved dioxygen is sufficiently high for the efficient conversion of alkyl radicals into peroxyl radicals. The initiated oxidation of organic compounds includes the following steps (see Chapter 2). 

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. 

Nitro compounds, like quinones, terminate chains in oxidizing compounds where hydroperoxyl radicals are formed. This was proved for the oxidation of polyatomic esters [37] and PP [38], Nitrobenzene retards the initiated oxidation of the following esters tetrapropionate of pentaerythritol, propionate of 2,2-dimethylbutanol, and dipropionate of 2,2-dimethylpro-panediol terminating chains by the reaction with peroxyl radicals [37]. The hydroperoxyl radicals were supposed to be formed as a result of the following reactions ... 

Recently an analogous mechanism for cyclic chain termination has been established for quinones [47], Quinones, which can act as acceptors of alkyl radicals, do not practically retard the oxidation of hydrocarbons at concentrations of up to 5 x 10 3 mol L 1, because the alkyl radicals react very rapidly with dioxygen. However, the ternary system, /V-phenylquinonc imine (Q) + H202 + acid (HA), efficiently retards the initiated oxidation of methyl oleate and ethylbenzene [47]. This is indicated by the following results obtained for the oxidation of ethylbenzene (343 K, p02 = 98 kPa, Vi = 5.21 x 10-7 mol L 1 s 1). 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

Aryl phosphites inhibit the initiated oxidation of hydrocarbons and polymers by breaking chains on the reaction with peroxyl radicals (see Table 17.3). The low values of the inhibition coefficient / for aryl phosphites are explained by their capacity for chain autoxidation [14]. Quantitative investigations of the inhibited oxidation of tetralin and cumene at 338 K showed that with increasing concentration of phosphite /rises tending to 1 [27]. 

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