Radicals molecules oxidized

It will be noticed that with chain-breaking antioxidants the additive will be consumed whilst if we assume that the AO2H molecule will regenerate A radicals the oxidation retarder is not effectively consumed. The difference between the two is illustrated schematically in Figure 7.4. 

Free-radical chain inhibitors are of considerable economic importance. The term antioxidant is commonly appUed to inhibitors that retard the free-radical chain oxidations, termed autoxidations, that can cause relatively rapid deterioration of many commercial materials derived from organic molecules, including foodstuffs, petroleum products, and plastics. The chain mechanism for autoxidation of hydrocarbons is ... 

Free-radical chain oxidation of organic molecules by molecular oxygen is often referred to as autoxidation (see Section 12.2.1). The general mechanism is outlined below. 

In reality, many other chemical and photochemical processes take place leading to a sort of steady-state concentration of O3 which is a sensitive function of height. To be accurate, it is necessary to include the reactions of nitrogen oxides, chlorine- and hydrogen-containing free radicals (molecules containing an unpaired electron). However, occurrence of a layer due to the altitude dependence of the photochemical processes is of fundamental geochemical importance and can be demonstrated simply by the approach of Chapman (1930). 

While many biological molecules may be targets for oxidant stress and free radicals, it is clear that the cell membrane and its associated proteins may be particularly vulnerable. The ability of the cell to control its intracellular ionic environment as well as its ability to maintain a polarized membrane potential and electrical excitability depends on the activity of ion-translocating proteins such as channels, pumps and exchangers. Either direct or indirect disturbances of the activity of these ion translocators must ultimately underlie reperfiision and oxidant stress-induced arrhythmias in the heart. A number of studies have therefore investigated the effects of free radicals and oxidant stress on cellular electrophysiology and the activity of key membrane-bound ion translocating proteins. 

Quantitative studies of such processes are of great interest for understanding the mechanism of chemisorption and a number of heterogeneous catalytic reactions, because it is superstechiometric (admixture) atoms (ions) of metals become active centers of adsorption of different particles (radicals, molecules) on metal oxides, or centers of catalysis. Such... 

In addition to hydrogen abstraction by peroxyl radical from another molecule, the reaction of intramolecular hydrogen transfer occurs in peroxyl radicals in oxidized hydrocarbons (see Chapter 2). 

Snyder, S. H., A novel neuronal messenger molecule in brain the free radical, nitric oxide, Ann. Neurol. 32 (1992), p. 297-311... 

As you have just seen, redox reactions are an essential part of your body s processes. However, these reactions can produce free radicals, which are highly reactive atoms or molecules with one or more unpaired electrons. Because they are so reactive, free radicals can oxidize surrounding molecules by robbing them of electrons. This process can damage DNA, proteins, and other macromolecules. 

In aqueous solution the electron transfer between (reducing) carbon-centered radicals or (oxidizing) hetero-atom-centered inorganic radicals and organic molecules often proceeds by covalent bond... 

Photolytic. Grosjean (1997) reported a rate constant of 1.87 x lO " cm /molecule-sec at 298 K for the reaction of 2-ethoxyethanol and OH radicals in the atmosphere. Based on an atmospheric OH radical concentration of 1.0 x 10 molecule/cm , the reported half-life of methanol is 0.35 d (Grosjean, 1997). Stemmier et al. (1996) reported a rate constant of 1.66 x 10 " cm /molecule-sec for the OH radical-initiated oxidation of 2-ethoxyethanol in synthetic air at 297 K and 750 mmHg. Major reaction products identified by GC/MS (with their yields) were ethyl formate, 34% ethylene glycol monoformate, 36% ethylene glycol monoacetate, 7.8% and ethoxyacetaldehyde, 24%. 

Alfassi, Z. B., and Benson, S. W., A simple empirical method for the estimation of activation energies in radical molecule metathesis reactions, Int. J. Chem. Kinetics S, 879 (1973). Allara, D. L., and Edelson, D., A computational analysis of a chemical switch mechanism. Catalysis-inhibition effects in a copper surface-catalyzed oxidation, J. Phys. Chem. 81, 2443 (1977). 

The phenylation of thiophene with benzoyl peroxide gave a considerable amount of 2,2 -dithienyl one suggested mechanism involved the formation of 2-thienyl radicals by oxidation, and their subsequent dimerization. More recent studies indicate that the 2,2 -dithienyl is formed through an initial addition of benzoyloxy radicals to the thiophene nucleus followed by dimerization of the resulting radical and loss of two molecules of benzoic acid (Scheme 15). 

Acidic products result from further oxidation of aldehydes (or ketones), again by a radical process. Oxidation of an aldehyde to a carboxylic acid in the presence of air involves a peroxy acid (compare peroxyacetic acid. Section 8.1.2). Finally, a reaction between the peroxy acid and a molecule of aldehyde yields two carboxylic acid molecules this is not a radical reaction, but is an example of a Baeyer-Villiger oxidation. Baeyer-Villiger... 

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

In this reaction (demonstrated in vitro), one of the two radicals is oxidizing while the other is reducing. In vivo, this reaction is catalyzed by one of several isoforms of an enzyme known as superoxide dismutase (SOD). As shown above, hydrogen peroxide may form as a result of the superoxide anion s dismutation reaction however, it may also be produced from a bivalent reduction of 02. The addition of the second electron leads to the formation of hydrogen peroxide, which is a powerful oxidizing agent. Due to the unpaired electrons in their outer shells, free radicals are favored to pair with other molecules during bimolecular collisions. 

It will be noted that Q does vary among the radicals in Table 29.2. Even more interesting is the fact that the molecules that form anion radicals upon reduction and cation radicals upon oxidation usually display larger proton splittings in the cationic species. Colpa and Bolton [9] proposed a more sophisticated equation based on the fact that negative charge in the p-type orbital will... 

Gersmann et al.74 suggested another mode of initiation, which proceeds by deprotonation of RH to a carbanion that transfers one electron to an oxygen molecule capture of oxygen produces a peroxy radical that oxidizes another molecule of the initial carbanion, the last two steps constituting a chain-propagation process. In hydrocarbon oxidations, Russell73 also showed that reactions of carbanions with 02 proceed via a two-step one-electron transfer pathway [Eqs. (29)-(32)]. 

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