Radical-chain reactions, inhibition

Although there seems little doubt that antimony trihalides play a chemical role in inhibition of free radical chain reactions in the flame zone, a comparison... 

The relative ease with which aryl benzyl sulfoxides undergo homolytic dissociation (Rayner et al., 1966) as compared to aryl benzyl sulfides or sulfones is supportive of this idea that ArSO radicals are easier to form than ArS or ArS02 radicals. Another interesting set of observations is the following. Booms and Cram (1972) found that optically active arene-sulfinamides ArS(0)NRPh (R = H or CH3) racemize thermally very readily at room temperature and that this racemization is the result of a free radical chain reaction (160) that is initiated by the dissociation of some of the sulfinamide into an ArSO and a PhNR radical (159). While the length of the inhibition... 

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 be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

Most important, the existence of an induction or inhibition period suggests a free-radical step in the decomposition of the thiophene ring. Further evidence for the free-radical nature of the reaction was obtained from experiments conducted under less severe conditions in order to isolate the initial ring-opened intermediate before subsequent loss of the one-carbon fragment. Efforts to isolate the initial decomposition product were unsuccessful. Apparently, the loss of the one-carbon fragment occurs rapidly, consistent with a free-radical chain reaction of some type. 

How can we keep our health against these reactive oxygen radicals Fortunately, vitamin C (hydrophilic), vitamin E (hydrophobic), flavonoids, and other polyphenols can function as anti-oxidants. These anti-oxidants are phenol derivatives. Phenol is a good hydrogen donor to trap the radical species and inhibits radical chain reactions. The formed phenoxyl radical is actually stabilized by the resonance effect as shown in eq. 1.8. Thus, phenol and polyphenol derivatives are excellent hydrogen donors to inhibit the radical reactions and, therefore, they are called radical inhibitors. 

Soluble Co compounds are generally employed in the autoxidation of hydrocarbons, i.e., the oxidation with O2 as the oxidant. In neat hydrocarbons, low concentrations of Co compounds accelerate the autoxidation since the Co2+/Co3+ couple is excellent for decomposing alkyl hydroperoxides and thus initiates free radical chain reactions. However, at high conversions, the Co may be deactivated by formation of insoluble clusters with side products of the hydrocarbon autoxidation. Moreover, high concentrations of a Co compound may actually inhibit the reaction because Co also terminates radical chains by reaction with ROO radicals ... 

The large group of inhibitors of free radical chain reactions are frequently used in combination with metal salts or organometallic stabilizers. They are amines, sulfur- or phosphorus-containing compounds, phenols, alcohols, or chelates. Aromatic phosphites at about 1 p.p.r. chelate have undesirable metal impurities and inhibit oxidative free radical reactions. Some of the more popular are pentaerythritol, sorbitol, melamine, dicyan-diamide, and benzoguanamine. Their synergistic effect is utilized in vinyl floors where low cost is imperative. 

Gas theories. — These attribute the retardant action to modification of the behavior of the volatiles (from the pyrolysis) by gases evolved from the decomposition of the retardant. Two suggested modes of action are (a) prevention of the formation of inflammable mixtures of air and volatile compounds (derived from the cellulosic material), by dilution with noninflammable gases derived from decomposition of the retardant, and (b) inhibition of free-radical chain-reactions in the flame, by introduction of decomposition products (from the retardant) that act as chain breakers. 

Xanthate 446 undergoes cyclization in the presence of camphorsulfonic acid via a radical chain reaction initiated by a small amount of lauroyl peroxide to give pyrroloimidazoles 449 in 56% yield. The use of an acid and anhydrous medium inhibits nucleophilic attack of the basic heterocycles at the xanthate moiety and allows radical reactions to occur. Fused heteroaromatic compounds can also be prepared directly from benzimidazole carrying an N-alkenyl substituent and xanthates by a tandem radical addition/cyclization to provide, for example, pyrrolobenzimidazole 453 in 57% yield (Scheme 106) <2002OL4345>. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

The allylperoxystannanes, like the allyl hydroperoxides, undergo the Schenck and Smith allylic rearrangements as illustrated in equation 14-87 for cholesterol derivatives.146 147 The rearrangements of the hydroperoxides are well established to be radical chain reactions 148,149 the rearrangements of the tin derivatives are inhibited by phenols, and it is assumed that they also proceed by a radical chain mechanism as illustrated for the Schenck rearrangement in Scheme 14-7.150 151 It will be noted that the mechanism... 

Historically, vitamin E has been recognized as necessary for neurological and reproductive functions, for protecting the red cell from hemolysis, and for prevention of retinopathy in premature infants. Inhibition of free-radical chain reactions of lipid peroxidation is the most thoroughly defined role of vitamin This occurs mainly within the polyun-... 

In contrast, the o-hydroxybenzophenone had no noticeable effect on the decomposition chonistry. Though it has been conmonly referred to as an ultraviolet absorber, it actually behaves more like a deactivator since the screen film containing it had a lower oxidation rate in Fig. 11 than the underlying clear film devoid of additives. Absorption can hardly account for the protective action of the stabilizer in thin films which are almost completely transparent to 300nm radiation. Neither can the hydroxybenzo-phenone be an important terminator of radical chain reactions because it is much less effective than other hindered enols in inhibiting oxidation in the dark. 

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. 

Some researchers pointed out that extracted humic materials might cause problems for the extraction and photolysis of probe (pollutant) molecules. First, the extraction efficiency may decrease, and second, humics are known to quench photochemical reaction. For example, Zepp R..G. et al. (1975) suggested that phenolic humic substances present on most inland waters could inhibit free radical chain reactions. Thus, quantum yields for direct photolysis of chlorinated compounds in the aquatic environment are not likely to exceed unity. 

Catechins can trap peroxyl radicals and thus suppress radical chain reactions and terminate lipid peroxidation. Catechins also inhibit metmyoglobin-initiated peroxidation of low-density lipoproteins (LDLs) and the consumption of a-tocopherol. Among tea catechins, EGCG is most effective in reacting with most reactive oxygen... 

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