Inhibitors free radicals

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Dichloroethane is produced by the vapor- (28) or Hquid-phase chlorination of ethylene. Most Hquid-phase processes use small amounts of ferric chloride as the catalyst. Other catalysts claimed in the patent Hterature include aluminum chloride, antimony pentachloride, and cupric chloride and an ammonium, alkaU, or alkaline-earth tetrachloroferrate (29). The chlorination is carried out at 40—50°C with 5% air or other free-radical inhibitors (30) added to prevent substitution chlorination of the product. Selectivities under these conditions are nearly stoichiometric to the desired product. The exothermic heat of reaction vapori2es the 1,2-dichloroethane product, which is purified by distillation. 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

Because they are acrylic monomers, alkyl cyanoacrylate esters still require the addition of radical polymerization inhibitors, such as hydroquinone or hindered phenols, to prevent radically induced polymerization over time [3j. Since basic initiation of alkyl cyanoacrylate monomers is the predominant polymerization mechanism, large quantities of free radical inhibitors can be added, with little or no effect on adhesive performance. 

Scheme 1. Influence of free radical inhibitor on the formation of products.

The fact that these reactions are catalyzed by free-radical initiators and inhibited by galvinoxyl (a free-radical inhibitor) " indicates that free-radical mechanisms are involved. 

Free-radical inhibitors do not slow the reactions, so no free-radical mechanism is involved. 

One of the present authors (31) has developed a series of additives which combine the features of both free radical inhibitors and flame retardants of the tetrabromophthalimide or chlorendic imide type with hindered phenol antioxidant structures such as the following compounds ... 

Direct free radical inhibitors suppress free radical formation by reacting with free radicals to form new inactive radicals (Reactions (1) and (2)) or chelating catalytically active transition metals to form inactive complexes ... 

Thus, vitamin C is able to replenish vitamin E, making the latter a much more efficient free radical inhibitor in lipid membranes. In addition, it has been suggested [9] that ascorbic acid can directly interact with the plasma membrane giving electrons to a trans-plasma membrane oxidoreductase activity. This ascorbate reducing capacity is apparently transmitted into and across the plasma membrane. 

The formation of 9,9 -biacridanyl and the suppression of amination by free radical inhibitors (78RCR1042) brings up the question of whether radical intermediates are involved... 

The advantage of using free radical inhibitors to facilitate the copolymerization of a bisbenzocyclobutene with a bismaleimide was first noted in a patent to Bartmann [78]. Subsequent to this, Corley in a series of patents described some detailed experiments on the copolymerization of bisbenzocyclobutenes with bismaleimides both with and without the addition of a free radical inhibitor [33, 34]. The structures of the bisbenzocyclobutenes used in this study are shown in Fig. 33. The bismaleimide component that was used was a mixture of three different bismaleimides in the molar ratio shown in Fig. 34. The individual bisbenzocyclobutenes were blended at elevated temperature with varying amounts of the bismaleimide composition. In some of the experiments, the free radical inhibitor phenothiazine was added at a 0.5 mole % level. The various monomer mixtures were then copolymerized using one of the cure schedules described in Table 14. The copolymers were then physically characterized using a variety of techniques. Table 14 shows the results obtained from copolymers... 

Haase and Dunkley (1969B) reported that although high concentrations of ascorbic acid in model systems of potassium linoleate were prooxidant, a decrease in the rate of oxidation was observed. Haase and Dunkley (1969C) further noted that certain concentrations of ascorbic acid and copper inhibited the formation of conjugated dienes, but not the oxidation of ascorbic acid, and caused a rapid loss of part of the conjugated dienes already present in the system. They theorized that certain combination concentrations of ascorbic acid and copper inhibit oxidation by the formation of free radical inhibitors which terminate free- radical chain reactions, and that the inhibitors are complexes that include the free radicals. 

To limit the radiolytic degradation of extractants, the influences of free-radical inhibitors have been measured. The addition of dimethoxybenzaldehydes (DMBA), particularly 3,5- and 3,4-DMBA, to the PUREX solvent could improve its stability and decrease its contamination (307). DMBA has a double effect, including a protective effect for the excited molecules of TBP (because of its low ionization potential), and the aldehyde radiolysis products could react with the HDBP present and therefore inhibit its complexing properties. 

The gas-phase elimination kinetics of ethyl oxamate, ethyl N,N-dimethyloxamate and ethyl oxanilate have been determined in a static reactor system, seasoned with allyl bromide and in the presence of a free radical inhibitor.12 These reactions are homogeneous, unimolecular and appear to proceed through moderately polar cyclic transition states. 

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... 

Figure 20 illustrates that a similar reduction in the rate of carbonyl formation also occurs under fluorescent blacklamp exposure. These results suggest that the antioxidant acts as a free radical inhibitor, reducing the over-all rate of photooxidation. The similarity in the relationship between the change in absorbance at 365 m/i, and carbonyl formation observed in the presence of the additives also lends further support to the previous conclusion that under exposures of this type, discoloration is caused by carbonyl products. 

The pyrolysis of 2-bromo-2-butene in a static system, with seasoned vessels, and even in the presence of a free radical inhibitor, was autocatalyzed by the HBr product109. However, under maximum catalysis with HBr gas, the reaction is molecular in nature and follows first-order kinetics. The overall rate coefficient was given by the following Arrhenius equation log kx (s-1) = (13.57 0.56)-(200.4 6.8) kJmol-1 (2.303R7)-1. The mechanism was suggested to involve a six-membered cyclic transition state as described in equation 22. 

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