Radicals, formation continued

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

It is important to emphasize that direet studies sueh as those earned out on the eyelopropylmethyl radieal ean be done with low steady-state eoneentrations of the radical. In the case of the study of the eyelopropylmethyl radical, removal of the source of irradiation leads to rapid disappearance of the EPR spectrum, because the radicals react rapidly and are not replaced by continuing radical formation. Under many conditions, the steady-state concentration of a radical intermediate may be too low to permit direct detection. Failure to observe an EPR signal, therefore, cannot be taken as conclusive evidence against a radical intermediate. 

A chain mechanism is proposed for this reaction. The first step is oxidation of a carboxylate ion coordinated to Pb(IV), with formation of alkyl radical, carbon dioxide, and Pb(III). The alkyl radical then abstracts halogen from a Pb(IV) complex, generating a Pb(IIl) species that decomposes to Pb(II) and an alkyl radical. This alkyl radical can continue the chain process. The step involving abstraction of halide from a complex with a change in metal-ion oxidation state is a ligand-transfer type reaction. 

Coadministration of dexrazoxane Chelation of iron in the heart, correction of iron dysregulation or mitigation of free radical formation Approved for use in patients who continue DOX above 300 mg/m2 or require another anthracycline after a prior exposure to 300 mg of DOX/m2... 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

There continues to be an increasing level of activity centered about the use of porphyrin catalysts for the epoxidation of alkenes of various configurations. For example, the sterically encumbered fra/w-dioxoruthenium(VI) porphyrin (26) was found to catalyze the epoxidation of a variety of alkenes in yields from fair to excellent e.g., 27 -> 28). Kinetic studies on a series of para-substituted styrenes point to a mechanism which proceeds via a rate-limiting benzylic radical formation. The high degree of stereoretention in cir-alkenes was attributed to steric crowding which prevents C-C bond rotation of the intermediate radical. This same steric bulk prevents the familiar side-on approach of the alkene substrate, so that a head-on approach is postulated <99JOC7365>. 

The following is not a proper product-forming step because it does not also produce a radical to continue the chain. It actually is a termination step, involving the coupling of two radicals. Formation of a major product... 

The addition of thiols to alkenes is a free radical reaction that is catalysed by formation of phenyl radicals from the dibenzoyl peroxide catalyst (12), and in which the R S- radical can continue the radical chain reactions as shown in Scheme 10. In this reaction, the initial cleavage of dibenzoyl peroxide (12) is facilitated by the relatively weak peroxide ( 0-0 ) bond the subsequently formed alkylthio (R S-) free radical is an electrophilic species which... 

Transient phenoxyl radicals Formation and properties in aqueous solutions 1137 TABLE 9. continued)... 

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