Radicals, delocalized, stability

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization Similarly electron delocalization stabilizes benzyl rad ical and weakens the benzylic C—H bond... 

The resulting radical is stabilized by electron delocalization and eventually reacts with either another inhibitor radical by combination (dimerization) or disproportionation or with an initiator or other radical. 

In orbital terms, as represented in Figure 11.10, benzyl radical is stabilized by delocalization of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the tt system of the ring. 

Although the resonance or delocalization stabilization of tri-phenylmethyl will be at its maximum only when the radical is completely planar, such a structure involves repulsion by adjacent ortho hydrogen atoms ... 

In order to circumvent the lack of selectivity in the cleavage of trialkylb-oranes, B-alkylcatecholboranes can be used as precursor of alkyl radicals. They are extremely sensitive towards oxygen and they react readily with alkoxyl radicals. It was clearly demonstrated by ESR that the perboryl radical intermediate resulting from the complexation of B-methylcatecholborane with the alkoxyl radical is stabilized by delocalization onto the aromatic ring (Scheme 30) [79]. 

The subject of ESR spectroscopy of heterocyclic radicals has been the topic of a previous review (74PMH(6)95). We consider here mainly the results which have appeared subsequently, and also draw attention to a fine review by Hansen (79AHC(25)205). Neutral heteroaromatic radicals require stabilization by delocalization of the odd electron, or they may be generated by continuous in situ photolysis, or trapped in an inert matrix, and examples of these techniques have been discussed (74PMH(6)95). 

Calculate CH bond dissociation energies in propene and in toluene, leading to allyl and benzyl radicals, respectively. (The energy of hydrogen atom is given at right.) Is bond dissociation easier or more difficult in these systems relative to bond dissociation in 3-ethylpentane (methyl CH) Examine spin density surfaces for allyl and benzyl radicals. Draw Lewis structures that account for the electron distribution in each radical. Does spin delocalization appear to stabilize radicals in the same way charge delocalization stabilizes ions ... 

HCL —R Draw resonance forms to show how the BHA radical is stabilized by delocalization of the radical electron over other atoms in the molecule. 

Conjugated compounds undergo a variety of reactions, many of which involve intermediates that retain some of the resonance stabilization of the conjugated system. Common intermediates include allylic systems, particularly allylic cations and radicals. Allylic cations and radicals are stabilized by delocalization. First, we consider some reactions involving allylic cations and radicals, then (Section 15-8) we derive the molecular orbital picture of their bonding. 

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. 

Just as delocalization stabilizes the allyl cation, anion, and radical, so too is the amide group stabilized by the conjugation of the nitrogen s lone pair with the carbonyl group. 

It is interesting to note that conventional carbon black supports promote the formation of peroxide, which then decomposes into radicals that attack the membrane. However, the role of graphitized carbon materials (such as CNTs) in peroxide formation is less clear. Smalley suggested that the curvy graphitic structure of CNTs deactivates free radicals by stabilizing them through enhanced delocalization. It would be worthwhile to determine whether the formation and fate of peroxide is any different between the carbon black and the CNT. At any rate, it is well known that the rate of formation of peroxide is greatly reduced by elimination of the carbon black support. Evidence of this is clear from the work we have done on carbonless electrodes (PTFE-bonded Pt black electrodes) and those with a hybrid structure. - " ... 

An antioxidant can be defined as a substance that significantly delays or prevents the oxidation of another substance even when present at a low concentration compared to the oxidizable substance of interest (Halliwell, 1990). For flavonoids, the antioxidant capacity has been linked to their radical scavenging ability (Bors et al., 1990). The flavonoids react readily with radicals such as hydroxyl ( OH), azid (N3 ) and peroxyl (ROO ), thereby forming stable flavonoid radicals. The flavonoid radicals are stabilized by extensive electron delocalization within the molecule, an ability that is crucial for the radical scavenging ability of antioxidants. The reaction... 

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