Decomposition rates, free radical initiators hydroperoxides

Very frequently, the phenomena described in the previous section are observed qualitatively but with considerably less sharpness, in the kind of autocatalysis associated with degenerate chain branching. Here, the active center involved in the chain branching step is not an active center at all but a relatively unstable intermediate product which, upon its decomposition or reaction provides active centers at a rate considerably faster than that of the original initiation. Thus the autocatalytic behavior can really be ascribed to a secondary initiation brought about by an intermediate product. This phenomenon happens frequently in the oxidation of hydrocarbons RH. At low temperatures, it is called autoxidation and it is autocatalytic because of the further decomposition into free radicals of hydroperoxides ROOH which are first produced in the oxidation (see p. 101). 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

The hydroxyl radical ( OH) generation from decomposition of ozone in alkaline pH mainly depends on the hydroperoxide free-radical initiating step (rate-determining step) in the chain reaction and the regeneration of the superoxide radical ion Oi ... 

The rate coefficients for hydroxyperoxide decomposition to free radicals is different from that for hydroperoxides. Therefore, addition of hydroperoxide to ketone changes the rate of free radical formation. This was first found for the system cyclohexanone—t-butyl hydroperoxide [168] with chlorobenzene as solvent. The rate of initiation increases with ketone concentration at a constant concentration of hydroperoxide. The... 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

Free radicals formed in polymers due to thermomechanical stress appear not only during the polymer use but also during the polymer processing and shaping to final products [46], The kind of initiation which prevails in a certain polymer depends not only on initial conditions of oxidation but also on the extent of a previous oxidation as well as on the occurrence of additional interactions among oxidation products. Increasing extent of oxidation is usually characterized by higher concentration of hydroperoxides which are secondary sources of initiation. The products of oxidation formed may alter the kinetics and mechanism of hydroperoxide decomposition so that the rate of initiation is the result of several mutually coupled processes. 

Oxidation reactions of hydrocarbons have a typical course. From the low rates, the reaction accelerates successively due to the consecutive formation of another source of free radicals which increases the rate of the primary initiation reaction. The amplification of the number of reactive free radicals is caused mainly by the decomposition of alkyl hydroperoxides, dialkyl and diacyl peroxides and peracids which are formed as intermediates in the oxidation reaction. 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

The use of chemical sensitizers such as benzoyl peroxide, cumene hydroperoxide, or azo-bis-isobutyronitrile, which decompose thermally to give free radicals in a convenient temperature range (i.e., 60 C to 150 C), makes it possible to study polymerizations over an extended temperature range. The form of the rate law with chemical initiations would be given by setting III = 2k (ln)< >i in Eq. (XVI.10.4). Here (In) is the initiator concentration, k I its specific rate constant of decomposition which can usually be measured independently, and is the efficiency with which its radicals initiate chains. The measure of t is subject to the difficulties already indicated in connection with the photolysis systems. ... 

As oxidation normally proceeds very slowly at the initial stage, the time to reach a sudden increase in oxidation rate is referred to as the induction period (6). Lipid hydroperoxides have been identified as primary products of autoxidation decomposition of hydroperoxides yields aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, known as secondary oxidation products. These compounds, together with free radicals, constitute the bases for measurement of oxidative deterioration of food lipids. This chapter aims to explore current methods for measuring lipid oxidation in food lipids. 

The antioxidant radical produced because of donation of a hydrogen atom has a very low reactivity toward the unsaturated lipids or oxygen therefore, the rate of propagation is very slow. The antioxidant radicals are relatively stable so that they do not initiate a chain or free radical propagating autoxidation reaction unless present in very large quantities. These free radical interceptors react with peroxy radicals (ROO ) to stop chain propagation thus, they inhibit the formation of peroxides (Equation 13). Also, the reaction with alkoxy radicals (RO ) decreases the decomposition of hydroperoxides to harmful degradation products (Equation 14). 

Considering the structural similarity between peracids and hydroperoxides, one might expect the kinetics of peracid decompositions to be complex. The rate law for the decomposition of perlauric acid is reported to be given by the sum of a first- and -order term in the peracid . Solely first-order dependence on perlauric acid has been claimed, but initial peracid concentrations were not varied to support this claim -Although the decomposition of peracids appears to be a free radical process in non-polar solvents, the reaction may be ionic in polar sol-vents - - ". ... 

In the derivation of the kinetic relations it was assumed that free radicals enter the particles one by one the initiation process just described satisfies this condition. This is not the case when radicals are formed by thermal decomposition of an oil-soluble initiator. Such decomposition produces pairs of radicals in the hydrocarbon phase. One would expect a pair of radicals, confined to the extremely small volume of a latex particle, to recombine rapidly. The kinetics of this type of polymerization have been described above. It is recalled here that the subdivision factor, z, and hence rate and degree of polymerization are smaller than 1 and decrease with a. These predictions from kinetic theory are in contradiction to experimental observations. Although some oil-soluble initiators, which are good catalysts in solution systems, are poor initiators in emulsion polymerizations—e.g., benzoyl peroxide—other thermally decomposing peroxides and azo compounds produce polymer in emulsion at rates comparable to those observed in polymerization initiated by water-soluble catalysts, where the radicals enter the particles one by one. Such is the case for cumene hydroperoxide, which at low concentrations yields a rate of polymerization per particle equal to that of a persulfate-initiated reaction. It must therefore be concluded that, although oil-soluble initiators may decompose into radical pairs within the particles, polymer radicals are formed one by one. The following mechanisms are consistent with formation of polymer radicals singly. 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

It is known that manganese salts cause oxidation of hydrocarbons, like cumene, by initiating free radical chain reactions. However, this is normally done by catalytic decomposition of trace amounts of hydroperoxides found in the hydrocarbons. In our case, the catalyst does not seem to decompose CHP, as demonstrated in an independent experiment (see above). If it did, the rate of decomposition should increase in time as the reaction progresses leading to an increase in the autoxidation rate. While we do observe for cumene an initiation period up to the accumulation of 3-5% hydroperoxide, from that point on up to greater than 50% CHP accumulation, the oxidation rate is constant. This initiation period may be due to surface activation of the catalyst. 

An investigation of the kinetics of the decomposition of cyclohexyl hydroperoxide at 60-70 in the presence of vanadyl acetylacetonate was recently carried out [355]. Cyclohexanol and cyclohexanone were formed in roughly a 1 1 ratio. The initial rate of decomposition was first order in initial concentrations of hydroperoxide and vanadium complex at [ROOM] <5 x 10 M and [VO(acac)2] < 1 x 10" M. The initial rate of decomposition changed from first order in [ROOH] to zero order giving evidence of complex formation prior to hydroperoxide decomposition. Using a chemiluminescence method the authors [355] concluded that only about 20% of the cyclohexyl hydroperoxide which decomposed gave free radicals. 

Sheldon has considered the competing process of homolytic decomposition of hydroperoxides during the epoxidation of olefins with tert-h xty hydroperoxide in the presence of molybdenum complexes. It was found that homolytic decomposition of the hydroperoxide is initiated by electron transfer reactions of Mo(V) and Mo(VI) complexes with the hydroperoxide giving rise to free radical species. Reaction rates and products of hydroperoxide decomposition were dependent on the solvent and on the presence or absence of an olefin. The rates and selectivities of epoxidation were highest in polychlorinated hydrocarbons and very poor in coordinating solvents such as alcohols or ethers [387]. 

The high effectiveness of the stabilizing action of organotin compounds, noted in many investigations and patents, can be explained by their ability to suppress free-radical chain processes. Dibulyltin dilau-rate, widely used as a stabilizer of polyvinyl chloride, proves to be an effective inhibitor of the radical decomposition of tertiary bulyl hydroperoxide, initiated by cobalt octoate [12]. 

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