Autoxidation radical chain mechanism

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). 

The weak chemiluminescence of Grignard compounds in air has been known since 1906. A radical chain mechanism similar to that of hydrocarbon autoxidation appears to provide the excitation energy of the emitting product. Until recently the relations between constitution and chemiluminescence in Grignard compounds were rather obscure j>-chloro-phenylmagnesium chloride was found to be the most efficient compound. 

In a free-radical chain mechanism we want to 1) produce a given product selectively, 2) simultaneously produce radical species which will further propagate the chain. Consider the autoxidation of m-chlorotoluene to m-chlorobenzoic acid in the three ways given on Figure 2. For the sake of argument, we initially start with MCPBA. We will also assume the free radical chain mechanism sequence does not contain a rate determining step. 

Scheme 7 Radical chain mechanism for the autoxidation of Zr(IV) alkyls...

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. 

Autoxidation, the oxidation of organic compounds by air, normally occurs via a radical chain mechanism. For example, cyclohexene undergoes allylic CH abstraction by an initiator, and the resulting cyclohexenyl radical reacts with O2 to give the corresponding hydroperoxy radical that abstracts an H from cyclohexene. In this case the final product is the allylic hydroperoxide. Conversion of ethers to the hydroperoxides is another familiar example. The conversion of cumene to phenol and acetone is a commercial application of the reaction (equation 9). 

Either the monomolecular or the dimolecular decomposition serves to feed new radicals into the reaction to initiate the chain reaction of autoxidation. These radicals may further react through different paths. They may follow a radical chain mechanism or other well-known radical reactions, such as coupling or disproportionation. 

Further possibilities for catalytic oxidation are realized in the well-known catalysis of the autoxidation of hydrocarbons and other substrates by salts of transition metals such as copper, cobalt, and manganese which exhibit more than one stable oxidation state and which catalyze oxidation through free radical chain mechanisms (16). 

This reaction falls into the category of autoxidation defined as the slow low-temperature oxidation of organic compounds by O2 via a radical chain mechanism, based on the fact that the oxygen molecule is a diradical... 

It has been suggested that autoxidation of saturated fatty acids and aldehydes occurs through a free-radical mechanism (13, 14). Supporting evidence of a radical chain mechanism was provided by Palamand and Dieckmann (15) who studied the autoxidation of hexanal. The reaction involves peroxycarboxylic acid as an intermediate (16) and probably proceeds via the mechanism shown in Figure 1. 

Transition-metal ions such as Fe(III), Cu(II), Co(II), Co(III), and Mn(II) have been shown to be effective homogeneous catalysts for the autoxidation of sulfur dioxide in aqueous solution. Hoffmann and coworkers have shown that Fe(III) and Mn(II) are the most effective catalysts at ambient concentrations for the catalytic autoxidation of S(IV) to S(VI) in cloudwater and fogwatet (Jacob and Hoffmann, 1983 Hoffmann and Jacob, 1984 Hoffmann and Calvert, 1985). Mechanisms for the homogeneous catalysis by Fe(lII) and Mn(II) that have been proposed include a free-radical chain mechanism, a polar mechanism involving inner-sphere complexation followed by a two-electron transfer from S(IV) to bound dioxygen, and photoassisted electron transfer. 

Free-Radical Chain Mechanisms. A frequently postulated initiation step for the free-radical autoxidation of S(IV) catalyzed by Fe(III) is as follows ... 

One-electron oxidation or reduction of saturated molecules frequently results in the generation of free radicals . The catalysis of certain free-radical reactions by ions or complexes of transition metals, such as (Tu, Co, and Mn, which exhibit variable oxidation states, is a consequence of this. Among such reactions are the autoxidations of hydrocarbons and other organic molecules (initially to hydroperoxides), which proceed by free-radical chain mechanisms in which the important propogation steps are ... 

Kinetic studies were carried out in order to determine the mechanism of the autoxidation reactions. The results indicate that the reactions do not proceed via the usual type of radical-chain mechanism involving hydroperoxides, and that not all metal acetylacetonates follow the same mechanism. A relatively simple mechanism has been proposed for the destructive autoxidation of iron(III) acetylacetone that postulates an intramolecular oxidation-reduction of the chelate with the formation of stable radicals, which are intercepted by highly reactive radicals produced by the decomposition of initiators. A triketone, 2,3,4-pentanetrione, is postulated as the intermediate from which most of the reaction products are derived (4, 5). 

The free radical chain mechanism of hydrocarbon autoxidation is well documented (2,8). The susceptibility of any substrate to autoxidation is determined by the ratio kp/(2k,) " - referred to as the oxidizability (9) - which determines the length of the propagating chain and, thus, the rate of the reaction (see Scheme 3). 

The free-radical substitution of H for OOH in alkanes is called autoxidation. ( Autoxidation is a misnomer, because the substrate is not oxidizing itself O2 is oxidizing the substrate ) Autoxidation proceeds by a free-radical chain mechanism. Note that the mechanism for oxidation includes a very rare radical-radical combination step in the propagation part. The radical-radical combination step doesn t terminate the chain in this particular reaction because O2 is a... 

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. 

Also potentially hazardous are compounds that undergo autooxidation to form organic hydroperoxides and/or peroxides when exposed to the oxygen in air (see Table 3.12). Especially dangerous are ether bottles that have evaporated to dryness. A peroxide present as a contaminant in a reagent or solvent can be very hazardous and change the course of a planned reaction. Autoxidation of organic materials (solvents and otho" liquids are most frequently of primary concern) proceeds by a free-radical chain mechanism. For the substrate R—H, the chain is initiated by ultraviolet... 

The autoxidation of Grignard reagents or dialkylmercurials proceeds by a free radical chain mechanism involving the attack of ROO upon RMgX (Walling and Buckler, 1955 Lamb et al., 1966) or R2Hg (Razuvaev et al., 1960 Aleksandrov et al., 1964). These reactions may well involve electron transfer or at least a transition state for the formal Sh2 substitution reaction resembling [1]... 

A complete mechanism for the autoxidation of alkylaromatic hydrocarbons by cobalt(n) in acetic acid has not been established,25 6 although a complex rate law has been determined for tetralin. 22 The reaction most likely proceeds by a fiiee radical chain mechanism in which the purpose of the cobalt ions is to provide a hi h steady state concentration of free radicals by catalysis of the decomposition of THP. The free radical nature of the autoxidation of tettalin with the colloidal CoPy catalysts is supported by experiments which showed inhibition of the reaction by 2,6-di-rerr-butylphenol and 2,6-di-rm-butyl-4-methylphenol, and by a shortening of the induction period and increase of the reaction rate when azobis(isobut nitrile) was added to the reaction mixture as a free radical initiator. 

The radical chain mechanism for autoxidation, using diethyl ether as the example. 

Lipid autoxidation is generally believed to involve a free- radical chain mechanism (1) initiation steps that lead to free radicals (R ), (2) propagation of the free radicals (R -I-O2 —> ROO, ROO -1-RH — ROOH-I-R ), and (3) termination steps R -H R R—R, R- ROO- ROOR, ROO ROO O 2 ROOR (or alcohol and carbonyl compound). The oxidation of lipids results in peroxides as primary oxidation products, which in turn degrade further to secondary oxidation products, including aldehydes, ketones, epoxides, hydroxy compounds, carboxylic acids, oligomers, and polymers. 

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