Benzylic radical

A similar reaction of benzyl radicals is observed in the thermolysis at 200 C and in the presence of powdered copper of N-benzylthiazolium... 

Reactions involving benzylic cations benzylic radicals and aUcenylbenzenes will be dis cussed m Sections 11 12 through 11 17... 

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... 

In orbital terms as represented m Figure 11 10 benzyl radical is stabilized by delo cahzation of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the rr system of the ring... 

FIGURE 11 10 The lowest energy tt molecular orbital of benzyl radical shows the interaction of the 2p orbital of the benzylic carbon with the TT system of the aromatic ring... 

The propagation steps in the formation of benzyl chloride involve benzyl radical as an intermediate... 

The unpaired electron in benzyl radical is shared by the benzylic... 

FIGURE 1111 Cham propagation in polymerization of styrene The growing polymer chain has a free radical site at the benzylic carbon It adds to a molecule of styrene to extend the chain by one styrene unit The new polymer chain is also a benzylic radical it attacks another molecule of styrene and the process repeats over and over again... 

Section 11 10 Chemical reactions of arenes can take place on the ring itself or on a side chain Reactions that take place on the side chain are strongly influ enced by the stability of benzylic radicals and benzylic carbocations... 

Benzylic carbon (Section 11 10) A carbon directly attached to a benzene nng A hydrogen attached to a benzylic carbon is a benzylic hydrogen A carbocation in which the benzylic carbon is positively charged is a benzylic carbocation A free radical in which the benzylic carbon bears the unpaired electron is a benzylic radical Benzyne (Section 23 8) The compound... 

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

Two radicals in which the radical site is exocyclic should be mentioned. These are the furan (165) and thiophene (166) analogs of the benzyl radical they long ago acquired trivial names, which are recognized by lUPAC because of their widespread use. 

The stabilizing effects of vinyl groups (in allylic radicals) and phenyl groups (in benzyl radicals) are very significant and can be satisfactorily rationalized in resonance terminology ... 

Allylic and benzylic radicals are also stabilized by both acceptor and donor substituents. As shown in Table 12.5, theoretical calculations at the MP2 level indicate that substituents at the 2-position are only slightly less elfective than 1-substituents in the... 

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 . ... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

The absolute rate of dissociation of the radical anion of /i-nitrobenzyl chloride has been measured as 4 x 10 s . The w-nitro isomer does not undergo a corresponding reaction. This is because the meta nitro group provides no resonance stabilization of the benzylic radical. 

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

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