Resonantly stabilized free radicals

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

At first, these highly reactive free radicals react with the antioxidant, but as the antioxidant is consumed, the free radicals react with other compounds. Hydrogens on methylene groups between double bonds are particularly susceptible to abstraction to yield the resonance stabilized free radical ( ) ... 

The second class of ring-forming reactions is recombination of resonantly stabilized free radicals, with subsequent rearrangement and ring formation. The most important such steps are believed to be recombination of two propargyl radicals,... 

NBS provides low cone. Br2 for free-radical bromination. Abstraction of one of the CH2 hydrogens gives a resonance-stabilized free radical product PhCHBrCH3. 

When a free radical reacts, it usually snatches an electron from the reactant, turning it into a free radical. This in turn will steal a single electron from another nearby molecule. A chain reaction ensues until two free radicals react together, effectively neutralizing each other, or alternatively, until an unreactive free-radical product is formed. Free radicals are said to be quenched by vitamin C, because the free-radical product — the ascorbyl radical — is so unreactive. As a result, free-radical chain reactions are terminated. Lipid-soluble vitamin E (a-tocopherol) works in the same way, in membranes rather than in solution, often in cooperation with vitamin C at the interface between membranes and the cytosol (the watery ground substance of the cytoplasm that surrounds the intracellular organelles). When vitamin E reacts with a free radical, it too produces a poorly reactive (resonance-stabilized) free-radical product, called the a-tocopheryl radical. Tocopheryl radicals can be reconverted into vitamin E using electrons from vitamin C. 

In 2007, Dellinger et al. (27A27) conducted and reported on the formation and stability of resonance stabilized free radicals of the type hypothesized by Pryor and his associates in the particulate phase of MSS. They concluded that the commonly observed free radicals in the particulate phase of MSS were not a surface associated semiquinone and were more likely an intrinsic, polymeric radical with a delocalized electron. The EPR signal observed by Pryor in the alcohol extract of the particulate phase of MSS may be from an extracted and autooxidized hydroquinone, not a particulate-phase-associated semiquinone radical. The semiquinone radical was observed in the particulate-phase MSS collected below 400°C and has a five-line spectrum with g 2.006. Semiquinone radicals were formed in the particulate phase of MSS only after aging. 

In addition to cation intermediates, radical intermediates can be used to introduce bromine or chlorine into a molecule. Both allylic and benzylic moieties form resonance stabilized free radicals that react with bromine or chlorine to give the corresponding halide. Allylic radicals are easily accessible from the corresponding allylic halides, particularly allyl iodides (secs. 13.3-13.5). Benzylic radicals are available from benzylic halides and also directly from the hydrocarbon, if it bears a benzylic hydrogen. Addition of bromine to 167 (in the presence benzoyl peroxide and photochemical initiation) gave benzylic bromide 168 in high... 

This is the initiation step of a free radical polymerization process. Free radical polymerizations, like all addition polymerization reactions, produce head to tail polymerization that is, the growing end of the polymer is the most stable possible radical. In the case of 1,3-butadiene the alkoxy radical attacks an end carbon and not an internal carbon. This is because attack of an end carbon forms a resonance stabilized free radical whereas attack of an internal carbon forms a radical with no such stabilization. Hence, the end carbon will be attacked by the alkoxy radical to form the more stcible species. This is shown as ... 

CH2 hydrogens gives a resonance-stabilized free radical product... 

If, however, one polymer free radical is resonance stabilized and the other is not, the resonance-stabilized monomer is preferentially added on to the resonance stabilized free radical, since, a new resonance species is formed. That is why styrene has a copolymerization parameter much greater than unity and vinyl esters have copolymerization parameters of much less than unity when these two monomers are copolymerized together. 

The carotenoids generally found in foods are linear zll-trans E form) polyenes formed from eight isoprenoid units. The structures of common carotenoids are shown in Figure 8.11. The linear conjugated polyene structure has the ability to delocalize an unpaired electron and hence the capacity to act to terminate free radical reactions with the production of resonance stabilized free radical structures. Thus, carotenoids may potentially (a) provide retinol and (b) act as antioxidants. 

Free radicals are highly reactive species that wreak havoc on cellular molecules. The antioxidant vitamin E, also called a-tocopherol, reacts with cellular free radicals and converts them to resonance-stabilized free radicals that are much less harmful. 

Ubiquinone is reduced in two successive one-electron transfer steps. The product of the first electron transfer, called a semiquinone, is a resonance-stabilized free radical. 

Many proteins and another coenzyme called flavin mononucleotide participate in this process, which involves five redox reactions. These redox reactions generate many resonance-stabilized free radicals. 

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