Peroxy radicals, chain termination

The decomposition may occur either uni- or bimolecularly to form alkoxy and peroxy radicals. These oxy radicals abstract a labile hydrogen from the hydrocarbon to produce either an alcohol or a hydroperoxide. The alkyl radical thus formed readily adds oxygen to reform a peroxy radical, and the process continues. When the autoxidation occurs In the absence of an antioxidant, the termination of the kinetic chain occurs chiefly by the combination of two peroxy radicals. This termination Is a source of alkoxy radicals which can undergo chain scission and give rise to volatile products and carbonyl groups. 

According to the results shown in Figure 1, the kinetic chain length of the photooxidation of isooctane is very low. Even for the lowest rate of radical initiation applied, I 10 M/h, the kinetic chain length of the non-inhibited photooxidation did not exceed a value of 1. Radical termination, therefore, seems to dominate over a peroxy radical chain reaction according to equation (5) in Scheme I. 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

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 effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

There is excellent agreement between the decay constants obtained by ceric ion oxidation of secondary hydroperoxides and the rate constants for chain termination in hydrocarbon autoxidation determined by the rotating sector. The agreement suggests that secondary peroxy radicals do not undergo many nonterminating interactions, so that most self-reactions of secondary peroxy radicals must be chain terminating. 

Chain-breaking antioxidants which interfere with the normal propagation processes may react with peroxy radicals, R02 or, more rarely, with the carbon radical, R. The antioxidant may react with the propagating radical by addition, by hydrogen transfer, or by electron transfer. The chain can be terminated directly, but more commonly a new radical is formed, which either continues the chain at a reduced rate or terminates a second chain. 

The methyl peroxy radical that is formed converts to H02 in the sequence for primary peroxy radicals shown in equations 30 and 31. The chain reaction is subject to termination steps that include the following ... 

Tertiary peroxy radicals also give slow termination rates (103-104 M-1 sec-1)91 because their dimerization yields an unstable tetroxide that either redissociates or decomposes, partly by a pathway producing new radicals that can continue chains.92 The scheme is outlined in Equation 9.37. Primary and... 

Termination in autoxidations is rather more complex than in the chain reactions we have considered so far. As we have noted briefly earlier (Section 9.2, p. 488), the peroxy radicals first combine to an unstable tetroxide, ROOOOR.124 The existence of these compounds when R is tertiary is inferred from isotope tracer studies,125 and the equilibrium 9.72 is observable by electron... 

In the early stages of autoxidations, hydroperoxide concentrations are low and chain initiation is inefficient. Under these conditions, Mn(II) and Co(II) can act as inhibitors by scavenging alkylperoxy radicals [reaction (278)]. Competition in the termination step between the usual bimolecular termination of peroxy radicals and their reaction with metal complexes can affect the chain length of the autoxidation. The expression for the chain length in a process involving bimolecular termination of peroxy radicals is... 

The phenomenon of catalyst-inhibitor conversion1 2,143,356 may be understood and critical concentration of metal can be deduced by reference to Eq. (280). If decomposition of the hydroperoxide is the source of initiation, it must be formed as rapidly as it is consumed to maintain a steady rate. If termination by metal complex predominates, a steady state occurs when the right-hand side of Eq. (280) equals unity. No oxidation will occur when this quantity is less than unity. Hence, catalyst-inhibitor conversion is observed as the metal concentration is increased to the point that the chain length becomes less than unity. If termination occurs by the bimolecular reaction of peroxy radicals, a chain length of less than unity will result in the depletion of the hydroperoxide until the rate of initiation has decreased to the point where the chain length is unity again. No inhibition is expected or observed. 

The reaction chains are terminated by the combination of allyl and peroxy radicals ... 

The first suggestion on the mechanism of the reaction between phenols and peroxy radicals emerged from experiments on the antioxidant effect of phenols, by Bolland and ten Have (1947a, b). These authors found a fair correlation between the increase of chain-terminating efficiency and the decrease of the redox potential of phenols and suggested a mechanism in which the phenolic hydrogen atom is transferred to the peroxy radical, i.e.,... 

Antioxidants such as tocopherols may be naturally present they may be induced by processes such as smoking or roasting, or added as synthetic antioxidants. Antioxidants act by reacting with free radicals, thus terminating the chain. The antioxidant AH may react with the fatty acid free radical or with the peroxy free radical,... 

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