Reactions of free radicals with molecular oxygen

3 - Reactions of free radicals with molecular oxygen 

Determine the kinetic parameters for the oxidation reaction of the secondary n-butyl radical. 

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Slagle, L, Park, P. Y., Heaven, M., and Gutman, D., Kinetics of Polyatomic Free Radicals. Reaction of Vinyl Radicals with Molecular Oxygen, J. Am. Chem. Soc., 106, 4356 (1984). 

Chemiluminescence, which is a low-intensity luminescence which develops as a result of conversion of a portion of the chemical energy into energy of electronic excitation of molecules and subsequent deactivation by the emission of light, was first detected in 1877 when lophine was oxidized by molecular oxygen in IM ethanolic KOH. In fact this chemiluminescence (broad band at 465-660 nm, Amax 545 nm) actually develops during deactivation of excited peroxide molecules formed in the reaction of lophine free radicals with molecular oxygen (77CHEll6o). 

Reaction of the organic free radical with molecular oxygen together with further hydrogen removal results in a chain reaction ... 

Chemistry of environment. The presence of oxidant atmosphere (usually air) determines the start of oxidative degradation from the start of irradiation. An inert atmosphere like vacuum or nitrogen does not provide any source of secondary reactions with radicals that appear during radiolysis. The diffusion of oxygen into irradiation material influences the distribution of oxidation products that takes a parabolic shape with the maximum amoimt at the both external sides and minimum is placed on the symmetry axe [14]. The penetration of molecular oxygen allows the reactions of free radicals with it and the peroxyl radical are formed. They are the initiators of further oxidation, which advances as a chain process. 

The CL analysis applied on the degradation of hydroxyl-terminated polybutadiene based polyurethane elastomers illustrates the effect of previous energetic treatment to which samples were subjected. The increase in the initial CL intensity and the shift of maximum CL intensity upon the shorter time are the effects of the initiation of oxidation by molecular scissions and, respectively, the reactions of free radicals with oxygen that leads to decrease in the material durability (Fig. 61) [06C1]. Another proof for the beginning of degradation by prior processing is the increased CL-emission at the start of the CL measurements. (Fig. 62). 

R. J. Blint, Calculation of the rate constant for the reaction of atomic hydrogen with molecular oxygen to form the free radical HO2, J. Chem. Phys. 73 765 (1980). 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

CASRN 56-40-6 molecular formula C2H5NO2 FW 75.07 Chemical/Physical Products identified from the oxidation of glycine and OH radicals (generated from H2O2/UV) in oxygenated water were oxalic acid, formic acid, and ammonium ions. In oxygen-free water, oxalic and formic acids were not produced, i.e., glycine oxidized directly to ammonium ions. The rate constant for the reaction of OH radicals with the zwitterion ion is 1.7 X 10 /M-sec and with the anionic form is 1.9 x 10 /M-sec (Vel Leitner et al., 2002). 

The fact that functionalization of polymers and small molecules is observed to occur predominately on terminal (methyl) carbon atoms does not imply that the oxyfluorination reaction is truly selective. Although the reaction mechanism has not been studied in detail, it is undoubtedly a free-radical process. Molecular oxygen reacts spontaneously with the fluorocarbon—hydrogen radicals generated by fluorine during the fluorination process. Acid fluorides are retained on terminal carbon atoms because they are stable in 1 atm of elemental fluorine. Hypofluorites, which may be short-lived intermediates of oxygen reactions with methylene radical sites along the carbon chain, are not observed in the functionalized polymers. It is probable that, if they are intermediates, they are cleaved and removed by the excess elemental fluorine. 

In the propagation sequence (Reactions 12.3 and 12.4), given an adequate supply of oxygen, the reaction between alkyl radicals and molecular oxygen is very fast and peroxyl radicals are formed (ROO ). These react with another fatty acid molecule producing hydroperoxides (ROOH) and new free radicals that contribute to the chain by reacting with another oxygen molecule. Hydroperoxide molecules can decompose in the presence of metals to produce alkoxyl radicals (RO ), which cleave into a complex mixture of aldehydes and other products, i.e., secondary oxidation products."... 

Sevilla MD, Becker D, Yan M (1990) The formation and structure of the sulfoxyl radicals RSO , RSOO , RSO2, and RSO2OO from the reaction of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen. Int J Radiat Biol 57 65-81 Horowitz A, Rajbenbach LA (1975) The free radical mechanism of the decomposition of alkylsulfonyl chlorides in hquid cyclohexane. J Am Chem Soc 97 10-13... 

The peroxysulfate radical, SO," is a key intermediate in the autoxidation of sulfite/bisulfite solutions, but is also the intermediate about which the least is known. It is formed subsequent to the one-electron oxidation of sulfite/bisulfite by the reaction of the sulfite radical with molecular oxygen (Reaction (29)). It is also generated in the metal ion-induced free radical decomposition of peroxy-monosulfate, H8O5" [107]. Its production in the oxidation of H8O5" by Ce(IV) has been confirmed by ESR spectroscopy [108], where a g factor of 2.0145 was... 

In many polymers luminescence above the glass transition temperature due to increased molecular motion increases sample light emission. However, some polymers behave exactly the opposite with decreased light intensity in samples exposed to temperatures above the glass transition. The rate determining step is the termination reaction of free radicals. For Nylon 6/6, increases in sample temperature increase chemiluminescence due to quenching of the excited carbonyl C=0. A model of the mechanism of chemiluminescence applied to Nylon 6,6 is reported in the Appendix. Based on this model, oxygen is never consumed in an autoxidation reaction. 

The presence of oxygen can modify the course of a fiee-radical chain reaction if a radical intermediate is diverted by reaction with molecular oxygen. The oxygen molecule, with its two unpaired electrons, is extremely reactive toward most free-radical intermediates. The product which is formed is a reactive peroxyl radical, which can propagate a chain reaction leading to oxygen-containing products. 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

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