Radicals resonance delocalization

In the resonance Raman spectra of GO0X (125), vibrational modes have been assigned to both the tyrosinate ligand (Tyr 495) as well as the tyrosyl radical (Tyr 272). The spectrum does not provide evidence for the speculation that the tyrosyl radical is delocalized onto the jr-stacked tryptophan residue (Trp 290) (126, 127). Recent results of high-frequency EPR measurement (30) on the apogalactose oxidase radical are also consistent with the radical spin density being localized on the modified Tyr 272 moiety only. 

The most common and also most effective mechanism of radical stabilization involves the resonant delocalization of the unpaired spin into an adjacent 7r system, the allyl radical being the prototype case. A minimal orbital interaction diagram describing this type of stabilization mechanism involves the unpaired electron located in a 7r-type orbital at the formal radical center and the 7r- and tt -orbitals of the n system (Scheme 1). 

Stabilized by resonance delocalization indeed, they are even more stable than tertiary radicals. In the presence of a suitable initiator, bromine dissociates to bromine atoms that will selectively abstract an allylic or a benzylic hydrogen from a suitable substrate, generating the corresponding allyl and benzyl radicals. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

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. 

Isoprene can be polymerized in the laboratory by a radical chain mechanism. As shown in the following equations, the odd electron of the initially produced radical is delocalized onto both C-2 and C-4 by resonance. Either of these carbons may add to another isoprene monomer to continue the chain reaction. If C-2 adds, the process is called 1,2-addition if C-4 adds, the process is called 1,4-addition. (This is similar to the addition of electrophiles to conjugated dienes discussed in Section 11.13 and the addition of nucleophiles to a,/8-unsaturated carbonyl compounds described in Section 18.10.)... 

Like carbocations, radicals can be stabilized by resonance. Overlap with the p orbitals of a tt bond allows the odd electron to be delocalized over two carbon atoms. Resonance delocalization is particularly effective in stabilizing a radical. 

Stabilization that takes place by delocalization of electrons in a it bonded system. Cations, radicals, and anions are often stabilized by resonance delocalization, (p. 163)... 

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. 

Answer The thermal homolysis of the weak oxygen-chlorine bond in r-butyl hypochlorite produces a r-butoxy radical that starts the chain. This radical will abstract a hydrogen atom from the benzylic methylene of 1-phenylpropane to give a resonance-delocalized benzylic radical, the most stable of all the possible alternatives. The propagation loop completes when the benzylic radical abstracts a chlorine atom from t-butyl hypochlorite and creates a r-butoxy radical to start the process over again. The products are 1-chloro-1-phenylpropane and r-butanol. 

An important factor which influences the rate of rearrangement is the extent of overlap between the SOMO and the bond to be cleaved. In conformationally mobile cyclopropylmethyl radicals rotation can occur to maximize this overlap. Substituents at the radical center which withdraw electron density, such as terf-butoxycarbonyl or nitro, dramatically decrease the rate of ring cleavage (Table 1), presumably because they reduce the overlap. Similarly, the rate of rearrangement is much slower in l-(cyclopropyl)allyl (3), l-(cyclopropyl)prop-2-ynyl (4), and a-cyanocyclopropylmethyl (5) radicals where resonance delocalization removes electron density from the a-carbon atom. 

Allylic carbocations, free radicals, and carbanions are resonance stabilized. In each case the stabilization is the result of delocalization of the positive or negative charge or the free radical. Resonance forms differ in the position of electrons and charge but not atoms. Every atom in an allylic carbocation, free radical, or carbanion possesses a p-orbital and the pi-electrons and charges or unpaired electrons are delocalized throughout these orbitals. 

Radicals substituted a to the amide linkage, 24, have been used in several studies to control stereochemistry in radical transformations, while radicals substituted a to esters, 25, and ethers, 26, have been used on a few occasions. Resonance structures for each of these radicals (A and B) can be written as shown in 24-26, with stabilization resulting from delocalization of the odd electron into the adjacent functional group. This resonance delocalization also restricts the geometry of these radicals, maximum delocalization being obtained when overlap between the radical and adjacent group is highest. 

Radicals can be generated from aromatic compounds by different methods and can be used for heterocycles synthesis. This is illustrated by the synthesis " in modest yield (45%) of benzoquinolones (phenan-thridones) starting with 2-aminobenzanilides (such as 4.43, Scheme 4.47). The amino group is converted to the stable (even when dry) dia-zonium fluoroborate (4.44) from which an aiyl radical is generated by action of metallic copper. The radical then adds to a double bond of the second benzene ring (Scheme 4.47) to form radical 4.45, which is resonance delocalized. An oxidative step (even just exposure to air) is then required to achieve the fully aromatic system of the phenanthridone (4.46). 

Resonance delocalization of allylic radicals means that bonding of the halogen can occur at either end of an allylic radical. With the allylic radical from propene the two possible substitutions are the same, but unsymmetrical allylic radicals lead to products that are... 

Benzylic radicals and benzylic cations are conjugated unsaturated systems and hoth are unusually stable. They have approximately the same stabilities as allybc radicals (Section 10.8) and allylic cations (Section 13-4). This exceptional stability of benzylic radicals and cations can be explained by resonance theory. In the case of each entity, resonance structures can be written that place either the unpaired electron (in the case of the radical) or the positive charge (in the case of the cation) on an ortho or para carbon of the ring (see the following structures). Thus resonance delocalizes the unpaired electron or the charge, and this delocabzation causes the radical or cation to be highly stabibzed. 

Interestingly, direct irradiation of ester- or carbo g lic acid-bearing VCP afforded five-membered lactones in low yields with unreacted VCP fScheme 11.39. Eq. 2). Formation of the cyclic lactone was presumably due to resonance delocalization of the biradical intermediate into the carbonyl group, revealing an O-centered radical species 53, which recombines to afford dihydrofuran 54. This dihydrofuran undergoes addition of water, followed by elimination of alcohol, to yield the cyclic lactone product 56 fScheme 11.401. ... 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

The eight atoms of the allyl radical lie in a plane, and all bond angles are approximately 120°. Each carbon atom is sp hybridized, and the three 2p orbitals participating in resonance delocalization of the radical are parallel to one another as shown in Figure 8.5. Like charged systems, in which a delocalized charge is more stable than a localized one, delocalized unpaired electron density leads to more stable structures than localized unpaired electron density. 

Autoxidation begins when a radical initiator, X, which is formed either by light activation of an impurity in the oil or by thermal decomposition of peroxide impurities, abstracts a doubly allylic hydrogen to form a radical. This radical is delocalized through resonance with both double bonds (1 and 2 in the following structure). 

The radical created from hydrogen abstraction from 0—H is stabilized by resonance delocalization... 

The sp hybridization of the phenyl ring changes the hybridization of the oxygen in the O—bond. The radical created from hydrogen abstraction from O—H is stabilized by resonance delocalization and many of the contributing structures have tertiary carbon radical character. 

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