Radical scavengers, organic

In natural water, the half-hves fall between these extremes. For example, the half-life of Lake Zbrich water (pH 8, 1.5 meq/LHCO ) is 10 min (27). The decomposition in natural water also can be initiated by trace metal ions, eg, Fe , promoted by impurities such as organic matter, and inhibited by HO radical scavengers, eg, HCO3, COg , HPO (25,28). 

The rate of aqueous ozonation reactions is affected by various factors such as the pH, temperature, and concentration of ozone, substrate, and radical scavengers. Kinetic measurements have been carried out in dilute aqueous solution on a large number of organic compounds from different classes (56,57). Some of the chemistry discussed in the foUowing sections occurs more readily at high ozone and high substrate concentrations. 

The use of monomers that do not homopolymerize, eg, maleic anhydride and dialkyl maleates, reduces the shock sensitivity of tert-huty peroxyesters and other organic peroxides, presumably by acting as radical scavengers, that prevent self-accelerating, induced decomposition (246). 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

Aromatic Amines. Antioxidants derived from -phenylenediarnine and diphenylamine are highly effective peroxy radical scavengers. They are more effective than phenoHc antioxidants for the stabilization of easily oxidized organic materials, such as unsaturated elastomers. Because of their intense staining effect, derivatives of -phenylenediamine are used primarily for elastomers containing carbon black (qv). 

The term free radical scavenger is often loosely used with little consideration of subsequent reactions. Virtually all organic molecules are scavengers of reactive species like hydroxyl radical. To be useful at reducing oxidative injury, a scavenger must react rapidly with longer lived oxidants and should not be converted... 

In this short review it is perhaps sufficient to indicate that in organic materials the primary processes and products of alpha and gamma radiation are usually fairly similar. However, major differences occur in the presence of so-called protecting additives, which are far less effective in the case of alphas. This is to be expected from the high concentration of radicals built up in the dense alpha track, far higher than in gamma or electron spurs. From the data on the effect of radical scavengers on the final product, the diameter of the alpha track can be estimated. 

Polyphenols can act as antioxidants by a number of potential pathways. The most important is likely to be by free radical scavenging, in which the polyphenol can break the radical chain reaction. Polyphenols are effective antioxidants in a wide range of chemical oxidation systems, being capable of scavenging peroxyl radicals, alkyl peroxyl radicals, superoxide, hydroxyl radicals, nitric oxide and peroxynitrate in aqueous and organic environments [121]. This activity is due to the ability of donating an H atom from an aromatic hydroxyl group to a free radical, and the major ability of an aromatic structure to support an unpaired electron by delocalization around the 7i-electron system. Phenolic acids... 

The decomposition of ozone is catalyzed by the hydroxide ion. Ozone dissociates in the presence of OH to H02°/02°. Further decomposition via the ozonide anion radical 03°7 HO,° results in the formation of OH° (see Figure 2-1, Part A, p. 11). They may react with organic compounds, radical scavengers (HC03, C032-) or ozone itself. 

A kinetic study of direct photolytic decomposition of organics showed that in aqueous solution organic substrates can be decomposed by UV radiation emitted by a polychromatic source. In UV/H202 photolytic destruction, the reaction between the organic compound and the hydroxyl radicals generated in the H202 photodecomposition is dominant. In addition, the kinetics must consider the reaction of hydroxyl radicals with the radical scavengers if they are present. The Arrhenius correlation was proposed to express the rate constants of the reaction between hydroxyl radicals and bentazone as a function of the temperature ... 

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