Radical resonance-stabilized allyl

In the case of cyclohexene, this leads to a resonance-stabilized allylic radical that then reacts... 

The mechanism is undoubtedly a free radical reaction that occurs very easily at the allyl site in propylene, forming the resonance-stabilized allyl radical. 

The bromine radical abstracts an allylic hydrogen atom of the cyclohexene, and forms a resonance stabilized allylic radical and hydrogen bromide. 

Butadiene reacts with chlorine under radical and ionic conditions to give both 1,2- and 1,4-addition products.259 The predominant formation of the trans isomer in radical 1,4-addition [Eq. (6.37)] was explained to result from the predominant transoid form of the starting compound and the configurational stability of the resonance-stabilized allylic radical intermediate ... 

Concerning the mechanism of Cr(VI) oxidations, initial attack of Cr03 to form a symmetric intermediate was proposed.677 Hydrogen atom or hydride abstraction from the allylic position leads to resonance-stabilized allylic radical or carbocation, respectively, which is eventually converted to the unsaturated carbonyl compound. 

The rearrangement of / ,y-unsaturated ketones can be viewed formally as the result of a preferential a-cleavage of the allylic bond to the resulting resonance-stabilized allyl radical with a widened range of recombination pos-[ sibilities. Thus ketone (16) photorearranges in /-butanol to the cyclopropyl... 

The resonance stabilized allyl radical will add new monomer molecules (vinyl acetate) and give in a first step a block copolymer ... 

NBS has proved to be especially useful in brominations at allylic positions because competition from the addition of bromine to the double bond is not a problem. Apparently, the fact that only a low concentration of Br2 is ever present in NBS brominations somehow inhibits the addition reaction. Examples are provided in the following equations. Note that the reaction is best if only a single type of allylic hydrogen is available to be abstracted. In addition, the resonance-stabilized allylic radical provides two sites that can abstract a bromine atom. If these two sites are different, a mixture of products is formed, as shown in the second example ... 

The general mechanism for allylic bromination shows that either end of the resonance-stabilized allylic radical can react with bromine to give products. In one of the products, the bromine atom appears in the same position where the hydrogen atom was abstracted. The other product results from reaction at the carbon atom that bears the radical in the second resonance form of the allylic radical. This second compound is said to be the product of an allylic shift. 

Like allylic cations, allylic radicals are stabilized by resonance delocalization. For example, Mechanism 15-2 shows the mechanism of free-radical bromination of cyclohexene. Substitution occurs entirely at the allylic position, where abstraction of a hydrogen gives a resonance-stabilized allylic radical as the intermediate. 

The overall mechanism of chromium(VI) allylic oxidation appears to craisist of removal of a hydrogen atom or hydride ion from the alkene, forming a resonance-stabilized allylic radical or carbocation, which is ultimately converted into die unsaturated ketone (Scheme 17). ... 

Unsaturated lipids are more easily oxidized than saturated ones because they contain weak allylic C-H bonds that are readily cleaved in Step [1] of this reaction, forming resonance-stabilized allylic radicals. Because saturated fats have no double bonds and thus no weak allylic C-H bonds, they are much less susceptible to air oxidation, resulting in increased shelf life of products containing them. 

The main processes thus involve the breaking of a bond p to the double bond, with formation of resonance-stabilized allylic radicals. 

By contrast (see Section 1.5d), the resonance stabilized allyl radical formed... 

The -CCli radical thus formed adds to the butadiene (step 3). Addition to one of the ends of the conjugated system is the preferred reaction, since this yields a resonance-stabilized allyl free radical. 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > -CH3. To this list we must also add the resonance-stabilized allyl and benzyl cations. Just as allylic radicals are unusually stable because the... 

The resonance-stabilized allylic radical intermediate has radical character on both the 1° and 3° carbons, so bromine can bond to either of these carbons producing two isomeric products. 

NBS generates bromine which produces bromine radical. Bromine radical abstracts an allylic hydrogen, resulting in a resonance-stabilized allylic radical. The allylic radical can bond to bromine at either of the two carbons with radical character. 

Hyperconjugation, which is at least partially responsible for the tertiary > secondary > primary stabilization order, is more important for cations than for radicals. The effect is large enough for tertiary cations to exceed resonance-stabilized allylic cations in stability (the reverse of the order for radicals). 

H abstraction is a competing process this leads to formation ofa resonance-stabilized, allylic radical. 

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