Inhibition of free radical reactions

Ammonia is evolved with the coal volatiles and may be therefore partially responsible for the reduction of NO that occurs in AFBC s. Because of the inhibition of free radical reactions within the bed this reaction is expected to be most important in bubbles and in the solid disengaging height above the bed. This importance of the NII3/NO reaction and its partial suppression by bed solids has been demonstrated by the injection of NH3 into... 

Another consequence of oxygen inhibition of free-radical reactions is that the adhesive surface exposed to air will contain a higher concentration of lower-molecular-weight species than the bulk of the adhesive. This can result in the surface being soft, tacky, or even remaining fluid. 

Potent inhibition of PGH synthase-dependent BP oxidation by antioxidants suggests that the quinones are products of free radical reactions (18 ). ... 

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 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. 

In 1961, a supposition was made about the great role of free radicals alien to normal cell in the development of some other diseases and about the feasibility of inhibiting the free-radical reactions by applying synthetic inhibitors to achieve a curing effect [9-11]. This supposition could be made only by physicochemists, first of all, by specialists in kinetics of the Semenov-Emanuel school, who imderstood the importance of not only (and not so much)of a change in the composition of the reaction components but also of then-physicochemical properties, i.e., when the same results may be obtained with different (in composition) components but with the one common physicochemical property - in this case, ability to react with free radicals. Therefore, synthetic compounds of the structure other than that of natural antioxidants may be used instead of (substitute) the latter ones in reactions with free radicals. 

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. 

Studies conducted in the presence of radical scavengers such as NO (refs. 383, 245, 409, 410), Oj (ref. 408) or H2S (ref. 246) have shown the importance of free-radical reactions in forming the products isobutane, 2,3-dimethylbutane, -butane, isopentane and others. The ethylene and propene yields are decreased by the presence of the scavengers owing to the disappearance of the fraction of these products that arises from disproportionation reactions. The products which are formed in the presence of inhibitors must arise from molecular or ion eliminations, ion-molecule reactions, excited molecule reactions or charge-neutralization reactions. Work on the inhibited radiolysis has led to a better understanding of the source of these products " . 

An important early method (1929) for the detection of gaseous free radicals was the metallic mirror technique of Paneth. Since then, many compounds have been used to determine the presence and extent of free radical reactions in the gas phase decomposition of organic compounds. The subject of inhibition in gas-phase reactions has been reviewed by Ashmore and by Gowenlock . 

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-... 

Characteristics of free radical reactions include acceleration by free radical initiators or light, inhibition by compounds such as phenols, kinetics including half-powers of concentrations, and the production of small quantities of by-products resulting from radical combination. 

We also have attempted to inhibit these free radical reactions with 2,6-di-t-butyl-4-methylphenol and found no effect on the formation of cyclohexanol or cyclohexanone using catalyst 4. This latter result strongly suggests that peroxyl, alkoxyl, or hydroxyl radicals are not intermediates in these reactions. The intermediacy of a putative oxo-manganese complex is further strengthened by the reaction of 1-4 with cyclohexene in the presence of TBHP or iodosylbenzene to provide cyclohexene epoxide and our proposed mechanism is shown in the Equation (2). w H... 

These investigators reported that in the presence of toluene, which serves to inhibit (suppress) free-radical reactions, the reaction obeys a first-order rate expression. 

These couples of oxidation protectors have the main role of delay on the progress of photochemical degradation by the oxidation inhibition of free radicals and by hydroperoxides decomposition. This type of compounds includes antioxidants (either for the inhibition of radical decay onto peroxyl radicals or for the decomposition of hydroperoxides), carbon black, ZnO (which is activated by UV radiation) or other compounds which are involved in the oxidation/reduction reactions. 

Figure 2. Enhancement of total butene yields from 100-torr ethylene with ionization potential of additive present in 10% concentration, 3 torr oxygen added when necessary to inhibit free radical reactions. The letter symbols indicate ionization potentials from Ref. 58 in parenthesis values in e.v.

Inhibit autoxidation of organic materials by interfering with free radical reactions that lead to incorporation of oxygen into macromolecules in a chain mechanism consisting of two interacting cyclical processes (Scheme II.1). 

Formulae for Induction Period r = r0f(x) of Hydrocarbon Oxidation (Table 14.4) Inhibited by Phenols of Group A Reactions are Nonstationary with Respect to ROOH t0 = f [lnH]0/vi0, / = k3/kdf vi0 is the Rate of Free Radical Generation on Reaction of RH with Dioxygen [33,38,45]... 

Metals and metal oxides, as a rule, accelerate the liquid-phase oxidation of hydrocarbons. This acceleration is produced by the initiation of free radicals via catalytic decomposition of hydroperoxides or catalysis of the reaction of RH with dioxygen (see Chapter 10). In addition to the catalytic action, a solid powder of different compounds gives evidence of the inhibiting action [1-3]. Here are a few examples. The following metals in the form of a powder retard the autoxidation of a hydrocarbon mixture (fuel T-6, at T= 398 K) Mg, Mo, Ni, Nb V, W, and Zn [4,5]. The retarding action of the following compounds was described in the literature. 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

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