Radicals oxygen and

Fig. 4. Mechanism of lipid peroxidation and its inhibition by vitamin E. Lipid peroxidation is initiated by generating a relatively nnreactive carbon-centered radical upon hydrogen abstraction by a hydroxyl radical (1). The fast formation (2) of the more reactive peroxyl radicals (ROO) ensures rapid attack of any peroxidizable substrate either by abstraction of a hydrogen atom (3a) or addition to a double bond (3b). The propagation is teiminated by mutual elimination of peroxyl radicals (4) or by suppression of free-radical formation in the presence of a-tocopherol (a-TOH) (5a). The tocopheryl radical is believed to be neutralized by ascorbic acid (AscAH) (5b) and radical oxygen, and a-tocopherol then re-enters the inhibition cycle.

This reaction creates further radicals (oxygen and hydroxy, 0 and HO ), which react with carbon monoxide exothermically. 

Azamar-Barrios J A, Munoz E P and Penicaud A 1997 Electrochemical generation of the higher fullerene radicals Tmoisture-free conditions and their observation by EPR J. Chem. Soc.,... 

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

The unique chemical behavior of KO2 is a result of its dual character as a radical anion and a strong oxidizing agent (68). The reactivity and solubiHty of KO2 is gready enhanced by a crown ether (69). Its usefiilness in furnishing oxygen anions is demonstrated by its appHcations in SN2-type reactions to displace methanesulfonate and bromine groups (70,71), the oxidation of benzyHc methylene compounds to ketones (72), and the syntheses of a-hydroxyketones from ketones (73). 

Atmosphere—Water Interaction. Although water is a very minor component of the atmosphere, less than 10 vol % of the atmosphere consisting of water, many important reactions occur ki the water droplets of cloud, fog, and rain. The atmosphere is an oxic environment ki its water phase, gigantic quantities of reductants, such as organic substances, Fe(II), SO2, CH SCH (dimethyl sulfide), and nitrogen oxides, are oxidized by oxidants such as oxygen, OH radicals, H2O2, and Fe(III). 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

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