Radical one-electron

I Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small amount of benzoyl peroxide catalyst to break the weak 0-0 bond. A benzoyloxy radical then adds to the C=C bond of ethylene to generate a carbon radical. One electron from the C=C bond pairs up with the odd electron on the benzoyloxy radical to form a C-O bond, and the other election remains on carbon. 

To understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation- and anion-radicals. One-electron addition to an organic molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to... 

In the case of the anion-radical of benzophenone, the effect of an added electron is not very specific It results in weakening of multiple bonds and strengthening of single bonds. Such an effect is usual for all organic anion-radicals. One-electron reduction of benzophenone, a fully conjugated ketone, yields a ketyl and results in the general bond loosening (Scheme 2.38). 

If X—Y is an electrophilic polar molecule such as CH,I, oxidative addition reactions tend to proceed by SN2 mechanisms involving two-electron transfer (Eq. 15.99) or via radical, one-electron transfer mechanisms (Eq. 15.100). 

These findings show that by dimerization to diphenol, structures are being formed which are less stable towards oxidation (radical one-electron transfer reactions), explaining dimerization by radical coupling during oxidative degradation from the energetic point of view. 

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

In the secimd imtiation step, a benzayloxy radical adds to ethylene to generate an alkyl radical. One electron from the carbon-cartmn double bond pairs up with (lie odd electron on the benroyloity initiator to form an O-C bond, and the -other electron remains on carbon ... 

Trans-resveratrol (trans-3,5,4 -trihydroxystilbene) is a non-flavonoid polyphenol found in grapes, mulberries, and other food prod-ucts. It is responsible for the antioxidant activity of red wine. Intake of moderate amount of red wine has been found to reduce the risk of cardiovascular diseases. In addition to antioxidant activity, resveratrol could inhibit platelet aggregation, and showed anticancer activity. The phenoxyl radicals of resveratrol produced during oxidation by hydroxyl radicals, one-electron oxidants, and peroxyl radicals showed absorption maximum at 410 nm (Fig. I). Reports indicate that trans-resveratrol is a better radical scavenger than vitamins E and C and its activity is similar to that of the flavonoids epicatechin and quercetin. From the comparison of the spectral and kinetic properties of the transients derived from trans-resveratrol and its analogues, it could be concluded that in the neutral and acidic solutions, the para-hydroxy... 

A radical X- abstracts a hydrogen atom from a C-H <5 bond to form H-X and a carbon radical. One electron from the C-H bond is used to form the new H-X bond, and the other... 

One electron comes from the radical. One electron comes from the jc bond. 

Biochemical Function. The best way to describe tocopherol s role is that of a lipid-soluble antioxidant. It protects unsaturated lipids from o gen-induced peroxide formation. There Is evidence for both free-radical one-electron chemistry (Fig. 8.20) and two-electron qui-none-hydroquinone chemistry (Fig. 8.21) (35). The oxidized/reduced glutathione system may he part of the system that regenerates reduced a-tocopherol. At one time it was thought that the preference for the 2-R stereoisomers indicated that the vitamin was part of a biochemical oaddation/reduction system, possibly as a coenzyme. So far that role for a-tocopherol has not been found. The current evidence points to the... 

The redox-mediated method is another way to generate carbon radicals. One-electron oxidation of neutral organic compounds having a carbon-heteroatom bond produces radical cation intermediates that decompose to give carbon radicals (Scheme 5.9a). Carbon radicals can also be generated by one-electron reduction followed by bond cleavage... 

Manganese superoxide dismutase (MnSOD) is a redox-active manganese enzyme that employs a mononuclear manganese ion at its active site. Discovered in 1970 (52), this enzyme catalyzes the dismuta-tion of superoxide (H02), to dioxygen and hydrogen peroxide, as shown in Scheme 2. Superoxide is the radical, one-electron-reduced... 

This organic acid is easily oxidized by one-electron transfer to produce the resonance-stabilized ascorbyl radical (AH ), which readily dismutates into a mixture of ascorbic (AH) and dehydroascorbic acid (DHA) (Figure 9,9). The ascorbyl radical is a strong acid (pK 0.45) and at pH 7 exists as a radical anion (A ). In reducing unsaturated lipid radicals one electron is transferred to the free radical from AH, producing the anion radical A. Aqueous solutions of AH contain low concentrations of the anion radical producing a 1 1 mixture of AH and DHA. 

At a non-active anode, the sorbed hydroxyl radicals ( physisorbed active oxygen ) from Reaction (1) can initiate free radical (one-electron) reactions of oxidizable substrates. Often, this leads to complete mineralization of organic materials (also called electrochemical combustion). 

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