Benzyl radical structure

The unpaired electron in benzyl radical is shared by the benzylic carbon and by the nng carbons that are ortho and para to it as shown by the spin density surface in Figure 119 Delocalization of the unpaired electron from the benzylic carbon to the ortho and para positions can be explained on the basis of resonance contributions from the fol lowing structures... 

Most stable Lewis structure of benzyl radical... 

The same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

Draw resonance structures for the benzyl radical, C6H5CH2-, the intermediate produced in the NBS bromination reaction of toluene (Problem 10.27). 

Sjsj2 reaction and, 377-378 Benzylic radical, resonance in, 578 spin-density surface of, 578 Benzylpenicillin, discovery of, 824 structure of, 1 Benzyne, 575... 

The amount of the resonance stabilization is similar to that for the benzyl radical. In radicals formed from monomers having C=0 or C=N groups conjugated with the carbon-carbon double bonds, the corresponding resonance structures... 

Otsu and Kuriyama designed photoiniferters which yield a highly reactive carbon radical and less reactive thiyl radical by photodissociation [169]. The former radical participates in propagation, and the latter acts only as a terminator. Bifunctional photoiniferters 8 as well as monofunctional 7 were prepared (see Eqs. 10 and 11 for the structures of 7 and 8). These photoiniferters dissociate only at the easily dissociable benzylic C-S bond to give a benzylic radical similar to the propagating poly(St) radical and the less reactive DC radical. 

Pyrroles, furans and thiophenes react preferentially with free radicals at the 2-position. Thus, reaction of pyrrole with benzyl radicals gives 2-benzylpyrrole. With triphenylmethyl radicals, pyrrole behaves like butadiene giving the adduct (163). /V-Methylpyrrole undergoes free radical benzoyloxyla-tion with dibenzoyl peroxide to give the 2-benzoyloxypyrrole (164) and 2,5-dibenzoyloxypyrrole (165). Furan, however, is converted in good yield to a mixture of cis and trans addition products analogous in structure to (163). 

Figure 4.32 Structures of a phenyl radical formed from the dissociation of a halo benzene, and of a benzyl radical from dissociation of a halomethylbenzene...

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 ... 

This approach is not expected to apply when there is considerable 7T-bonding between the substituent and the ring. Thus the anion of nitrobenzene probably has a structure more analogous to that of the benzyl radical rather than to that of the benzene anion, and the results (Maki and Geske, 1960) are indeed closer to those expected for such a model than for a model in which the impaired electron is placed in the symmetric e2 level of benzene. 

Anthracene is a polycyclic conjugated structure that cannot be fragmented into smaller units. The oxidants each have preferred points of oxidation on the conjugated rings, but the 9 and 10 positions are favored because of symmetry and stabilization of benzylic radicals. Rate constants for oxidation of anthracene by ROz, 102, and HO have been measured in solution or in vapor kox(R02) is 60 M 1 s 1 in chlorobenzene at 60°C (Mahoney, 1965) ... 

When an O-acyl ester (2) derived from phenylacetic acid is irradiated, a beautiful hyperfine structure of a benzyl radical can be observed by ESR [6]. This can be explained by the fact that the rate constant for the reaction of the formed R and O-acyl esters (2) is about 106 s"1 and it is rather slower than that of decarboxylation of RCO2 (eq. 8.3) [7, 8]. Generally, under photolytic conditions, approximately 100% of the chain reaction occurs, and under benzene refluxing conditions, approximately 80% of the chain reaction and approximately 20% of the caged reaction takes place. 

High energy radiation generates ionic and free radical intermediates in styrene, and both ionic and radical polymerizations take place simultaneously. Therefore, the observation of neutral radicals related to the radical polymerization had been expected. The absorption of radicals with a maximum at 320-330 nm was actually observed in the pulsed styrene and a-methylstyrene it decayed over a time interval of several hundred microseconds without being affected by the presence of water [12 14]. This absorption was attributed for the main part to a radical with a benzyl-type structure. The similar absorptions were observed in the cyclohexane solutions [21], Swallow suggested that this intermediate would be formed by an ionic process, probably by the protonation of a... 

Much of Janzen s criticism has been justified the spin distribution in benzyl radicals themselves shows little substituent dependence44 dimeth-ylanilinium radicals, by Walter s structural criterion of type S, have proved to be of type 045 the original data on triarylaminium and triarylmethyl radicals, used to exemplify type S, have proved ambiguous45-47 authenticated S type behavior has been found only in a small group of acyclic aminyl... 

One resonance structure of the benzyl radical (PhCH2 ) is shown below ... 

This work and related studies provided the basis for the stereoelectronic model for homobenzylic ether cleavage in Fig. 3.1. This model includes overlap of the benzylic carbon-carbon bond with the SOMO of the aromatic ring (structure 13), thereby stabilizing the benzylic radical upon cleavage. Additionally, overlap of a heteroatom lone pair and the benzylic o orbital was shown to be necessary for cleavage (structure 14). 

Draw as many re nanee structures as you can for the benzyl radical. C HjCHa. the intemnediate produced in the NBS bronsination reacticHi of toluene (ProWem. 

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