Radical-chain reactions, inhibition termination

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... 

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. 

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 ... 

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. 

As noted, the transient nature of most free radical species is a major consideration in ESR studies of free radicals. Free radical chemistry [77] involves an initiation step in which the free radicals are formed, often followed by one or more propagation (chain) reactions before termination. Because most radical-radical termination reactions are fast, the majority of free radicals decay rapidly by self reaction, i.e., they are transient even in the absence of another species. (In non-transient, i.e., persistent, radicals the radical center is sterically hindered, thereby inhibiting self-reaction.) A comment on terminology may be appropriate at this point many transient radicals are frequently described as stable or unreactive, which can lead to some confusion. The source of this confusion is that reactivity and stability are often used to denote... 

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. 

Lipid peroxidation (see Fig. 17.2) is a chain reaction that can be attacked in many ways. The chain reaction can be inhibited by use of radical scavengers (chain termination). Initiation of the chain reaction can be blocked by either inhibiting synthesis. of reactive oxygen species (ROS) or by use of antioxidant enzymes like superoxide dismutase (SOD), complexes of SOD and catalase. Finally, agents that chelate iron can remove free iron and thus reduce Flaber-Weiss-mediated iron/oxygen injury. 

Ruhho, H., Parthasarathy, S., Barnes, S., Kirk, M., Kalyanaraman, B., and Freeman, B. A., 1995, Nitric oxide inhibition of hpoxygenase-dependent hposome and low-density hpoprotein oxidation termination of radical chain propagation reactions and formation of nitrogen-containing oxidized hpid derivatives, Arclr. Biochem. Biophys. 324 15-25. 

Wall termination reactions immediately introduce a complexity to all chain reactions, namely, that the overall reaction rate can be a strong function of the size of the reactor. In a small reactor where the surface-to-volume ratio is large, termination reactions on surfaces can keep the radical intermediate pool small and thus strongly inhibit chain reactions (nothing appears to happen), while in a large reactor the surface-to-volume ratio is smaller so that the termination rate is smaller and the effective rate increases by a large factor (and the process takes oft). 

In the development of effective catalytic oxidation systems, there is a qualitative correlation between the desirability of the net or terminal oxidant, (OX in equation 1 and DO in equation 2) and the complexity of its chemistry and the difficulty of its use. The desirability of an oxidant is inversely proportional to its cost and directly proportional to the selectivity, rate, and stability of the associated oxidation reaction. The weight % of active oxygen, ease of deployment, and environmental friendliness of the oxidant are also key issues. Pertinent data for representative oxidants are summarized in Table I (4). The most desirable oxidant, in principle, but the one with the most complex chemistry, is O2. The radical chain or autoxidation chemistry inherent in 02-based organic oxidations, whether it is mediated by redox active transition metal ions, nonmetal species, metal oxide surfaces, or other species, is fascinatingly complex and represents nearly a field unto itself (7,75). Although initiation, termination, hydroperoxide breakdown, concentration dependent inhibition... 

Some organic and inorganic substances react with OH0 to form secondary radicals which do not produce H027020. These inhibitors (or scavengers) generally terminate the chain reaction and inhibit ozone decay. 

Generally speaking, the direct ozonation is important if the radical reactions are inhibited. That means that the water either does not contain compounds that initiate the chain reaction (initiators) or it contains many that terminate the chain reaction very quickly (scavengers). With increasing concentrations of scavengers the mechanism of oxidation tends to the direct pathway. Therefore, both inorganic carbon as well as the organic compounds play an important role. 

The qualitative features of paraffin pyrolysis, on the other hand, are reasonably well understood. The decomposition is, in general, initiated by rupture of C-C bonds, carried by chains of hydrogen atoms, methyl radicals, and to some extent ethyl radicals, and terminated by assorted radical recombinations. Product inhibition occurs through the reaction... 

Autoxidation can be inhibited or retarded by adding low concentrations of chainbreaking antioxidants that interfere with either chain propagation or initiation (286). Chain-breaking antioxidants include phenolic and aromatic compounds hindered with bulky alkyl substituents. Common synthetic chain-breaking antioxidants used in food lipids include butylated hydroxyanisole (BHA), butylated hydroxyto-luene (BHT), ferf-butyUiydroquinone (TBHQ), and propyl gallate (PG). This class of antioxidants react with peroxy free radicals to terminate reaction chains. The antioxidant radical (A ) formed in Equation 5 should be relatively stable and unable to initiate or propagate the oxidation chain reaction. 

The chain reaction sustains itself until it is terminated by direct combination of H and Cl, probably at the walls of the containing vessel. Such a reaction therefore tends to propagate itself without further encouragement until the reactants are exhausted. There are many examples of chain reactions in organic chemistry, where the active intermediate is often a free radical such as CH 3. Sometimes these are undesirable e.g. in the premature oxidation of hydrocarbons under pressure, which causes knocking in internal combustion engines) and it is necessary to inhibit them by suitable additives which operate by terminating the chains. 

They seem to interfere, by slow reactions, with propagating macroperoxy radicals secondly, they might be capable of reducing the number of initiated chain reactions by fast inhibition of peracid radicals produced by non-terminating recombination of primary macroperoxy radicals within initiated radical pairs. 

Each aromatic amine molecule, InH, terminates many free radical chains in autooxidation of alcohols and amines due to the ability of oxyperoxy and aminoperoxy radicals to oxidize InH as well as to reduce In to InH (JO. However, the coefficient of inhibition, f > 2, can be very often observed in oxidizing hydrocarbons too (2 ). Therefore, some reduction of aminyl radicals to InH proceeds in oxidizing hydrocarbons. To ellucidate the ways of such reduction we have studied the products and kinetics of the reactions of diphenylaminyl radical In. ... 

---