Free radical stabilization

Free radicals like carbocations have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that release electrons Consequently the order of free radical stability parallels that of carbocations... 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

The new free radical stabilizes itself by splitting out a simple molecule such as olefin or CO ... 

Free radicals, stabilization by sulphinyl and sulphonyl groups 533-535 Furans 638, 679, 840... 

Both -NH2 and -COOH groups are hydrophilic esterifying the -COOH group under acidic conditions would render surfactant properties to the final derivative (Figure 3). If both the -COOH and -NH2 functionalities of the amino acid are derivatized, making it a secondary amino compound, then it can be used as fuel stabilizer, although the exact mechanism of -NH- as free radical stabilizer is still uncertain. 

In a paper on free radicals published in 1929, Ingold and H. Burton suggested that the forces responsible for free radical stability and for the peculiarities of benzene valency arise from the delocalization of electrons permitted by alternative valency structures. 38 Ingold was influenced, he later said, by... 

HC 1401-350 residue may not show the effect of bond breaking because of its low aliphatic hydrogen which must be available for free radical stabilization. 

The results observed in this system unambiguously support free-radical stability as a significant governing principle. By far the major product (62) is derived by attack on CS a minor product (63) is due to attack on C6 and hydrogen migration is effectively eliminated. These results show clearly that steric hindrance is of some importance in these reactions, although the transition states for some of these reactions q>pear to be remarkably early. ... 

If you recall that combustion is a free radical process, we can easily see why cyclic and branched alkanes bum more easily (and more smoothly) than straight-chain alkanes. The reason is that more stable free radicals are formed. This results in less knocking and a higher octane rating. Examples of free radical stability are the following ... 

In thermal free radical cracking, fi agmentation to small molecules occurs readily. In catalytic cracking carbocations rearrange more because the differences in carbocation stabilities are greater than the differences in free radical stabilities. 

TABLE 5.3 0 values for some R—H bonds 73 Free-radical stability is in the reverse order... 

This secondary carbon position offers the best combination of free radical stability and ability to approach the enzyme s reactive site. This addition product could be further transformed to yield 2-carboxy-substituted compounds (So and Young, 1999), derivatives that are subsequently used in pathways involving fatty acids. 

This last possibility appeared to offer the most fruitful line of investigation. Therefore, we decided to determine what kinds of structural features would confer unusual free radical stability on simple model phenols which had been oxidized. Several phenols were oxidized in benzene solution by one-electron oxidants such as Pb02, alkaline ferricyanide,... 

Since the order of free-radical stabilities falls in the order 3° > 2° > 1°, product stability would dictate that cyclization should preferentially occur to give die more stable secondary radical—a six-membered ring in reaction (9.1) (path a) and a seven-membered ring in reaction (9.2)(path a). 

Resonance effects, on the other hand, can significantly affect the regiochem-istry of the cyclizadon. Resonance delocalization of the unpaired electron of a free radical stabilizes that radical. This is why the allyl radical is much more stable than the //-propyl radical. Thus, if a double bond is substituted with a group capable of providing resonance stabilization to a free radical, it undergoes free-radical addition much more readily than a double bond which cannot provide such resonance stabilization. 

Vitamin E has been shown, in a wide range of studies, to be capable of affording considerable protection to cells exposed to free radical attack, either from pathological or drug-induced sources. In injured nervous tissue, vitamin E probably exerts its protective effect by scavenging free radicals, stabilizing cellular membranes and quenching the cascade of biochemical events that... 

The behavior of dibutyltin bis(dodecylmercaptide) on reaction with 4-chloro-2-pentene proved interesting (Figure 3). With only the two reactants in chlorobenzene, virtually no reaction took place up to 5 hours. However, the addition of dibutyltin dichloride resulted in a rapid reaction. Furthermore, the addition of a few milligrams of azobisisobutyro-nitrile eliminated any induction period. This latter consequence is not interpreted to result from a free radical stabilization mechanism, but it is presumed to be caused by free radical-catalyzed hydrogen chloride elimination, resulting (by neutralization with the stabilizer) in the formation... 

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