Generation of reactive free radicals

Scheme 10.3 Generation of reactive free radicals with the aid of type II initiators, exemplified by the reaction of a triplet-excited diaryl ketone with a tertiary amine.

Scheme 10.8 Generation of reactive free radicals during the absorption of light by titanium dioxide.

The reactions mentioned so far all take place with the generation of a free radical of high reactivity which is capable of sustaining the chain reaction. 

As strong metal ion chelators due to their catechol structure, tea flavonoids are able to bind and thus decrease the level of free cellular ferric and ferrous ions, which are required for the generation of reactive oxygen radicals via the Fenton reaction (Yang and Wang, 1993). 

Previous work in our laboratory (3) and in others (4) has established that the primary photoprocess in a variety of excited carbanions involves electron ejection. This photooxidation will generate a reactive free radical if recapture of the electron is inhibited. Parallel generation of these same carbon radicals by electrochemical oxidation reveals an irreversible anodic wave, consistent with rapid chemical reaction by the oxidized organic species (5). Little chemical characterization of the products has been attempted, however (6). 

Irradiation of powdered titanium dioxide suspended in solutions containing aromatic compounds and water under oxygen has recently been shown to induce hydroxylation of aromatic nuclei giving phenolic compounds and oxidation of side chains of the aromatic compounds (50-55). These reactions have been assumed to proceed through hydroxyl and other radical intermediates, but the mechanism for their generation, whether reactive free radicals result from oxidation of water, from reduction of oxygen, or from oxidation of the substrates on the surfaces of the excited titanium dioxide, has not been clear. 

It is now accepted that an efficient reaction requires the generation of a free radical or similar reactive species (5), in a position which permits transfer of the hydrogen atom through a cyclic transition state (6) comprising six atoms, four of which are carbon. Cyclic transition states with more or fewer atoms are apparently too strained to permit useful reaction. The reactive atom (X) may be 0 or N, and various methods have been devised for initiating the reaction in each case. If hydrogen transfer leads to a-CH2 radical (7), this may undergo reaction either with a suitable species from the solution, with another free radical, or with a reactive intramolecular site. 

The reactions mentioned so far all take place with the generation of a free radical of high reactivity which is capable of sustaining the chain reaction. However, other molecules exist which form free radicals of such high stability that they effectively stop the chain process. These molecules are called retarders or inhibitors the difference is one of degree, retarders merely slowing down the polymerisation reaction while inhibitors stop it completely. In practice vinyl monomers such as styrene and methyl methacrylate are stored with a trace of inhibitor in them... 

Uetrecht JP (1992) The role of leukocyte-generated reactive metabolites in the pathogenesis of idiosyncratic drug reactitms. Dmg Metab Rev 24 299—366 Uetrecht JP (1995) Myeloperoxidase as a generator of drug free radicals. Biochem Soc Symp 61 163-170... 

For optimization of the hydrogen abstraction grafting process, the soluble polymer must be carefully chosen and should be used with an initiator that generates very reactive free radicals (e.g., benzoyl peroxide). Adventitious hydrogen abstraction and grafting can also occur. Many dispersions, in which steric stabilization is claimed to arise from the adsorption of simple soluble polymers, may thus actually be stabilized by graft copolymers formed in situ during the preparation of the dispersions [3.64],... 

Initiation stage of free radical mechanism for polyethylene showing generation of reactive free radicak. R represents the active initiator and is an unpaired electron. 

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