Electron delocalization benzylic carbocations

Section 11 14 Benzylic carbocations are intermediates in SnI reactions of benzylic halides and are stabilized by electron delocalization... 

Figure 11.12 Resonance forms of the allyl and benzyl carbocations. Electrostatic potential maps show that the positive charge (blue) is delocalized over the ir system in both. Electron-poor atoms are indicated by blue arrows.

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

A carbocation is strongly stabilized by a C substituent (Figure 7.1c) through n-type interactions which involve substantial delocalization into the substituent. The LUMO energy is relatively unchanged, but the reactivity of the electron-deficient center toward attack by nucleophiles is reduced because the orbital coefficients are smaller. Allyl and benzyl carbocations are prototypical of C -substituted carbo-cations. The effects of substitution are cumulative. Thus, the more C -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. A prime example is triphenyl carbocation. 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

The rules lead us to an incorrect prediction of the reaction product because they do not take electron delocalization into consideration. They presume that both carboca-tion intermediates are equally stable since they are both secondary carbocations. The rules do not take into account the fact that one intermediate is a secondary alkyl car-bocation and the other is a secondary benzylic cation. Because the secondary benzylic... 

The addition of a proton to the alkene forms a secondary alkyl carbocation. A carbocation rearrangement occurs because a 1,2-hydride shift leads to a more stable secondary benzylic cation (Section 4.6). It is electron delocalization that causes the benzylic secondary cation to be more stable than the initially formed secondary carbocation. Had we neglected electron delocalization, we would not have anticipated the carbocation rearrangement, and we would not have correcdy predicted the product of the reaction. 

Since cations B, F and G do not undergo electron delocalization, they are less stable than the other cations Cations A, C, D and E have the positive charge distributed amongst four carbons, three of which are the same (ring carbons) Since carbocation stability follows the order 3 > 2° > 1°, cation C is the most stable one. Cation C is a tertiary benzylic cation, cations A, D, and E are either secondary or primary benzylic cations. Hence,... 

Benzylic carbocations, radicals, and anions resemble their allylic counterparts in being conjugated systems stabilized by electron delocalization. This delocalization is describable in resonance, valence bond, and molecular orbital terms. 

Benzylic and allylic halides readily undergo SnI reactions as well, because they form carbocations that are stabilized by electron delocalization (Section 8.13). 

Richard interprets these measurements as implying an increase in delocalization of charge and increase in double bond character at the benzylic carbon atom of the carbocation as the number of electron withdrawing fluorine substituents increases. This is consistent with a changing balance of contributions of the valence bond resonance forms 59 and 60. 

The stability of the radicals depends on the nature of the atom that is the radical centre and on the electronic properties of the groups attached to the radical. As in the case of carbocations, the order of stability of the free radicals is tertiary > secondary > primary > methyl. This can be explained on the basis of hyperconjugation as in the case of carbocations. The stability of the free radicals also increases by resonance possibilities. Thus, benzylic and allylic free radicals are more stable and less reactive than the simple alkyl radicals. This is due to the delocalization of the unpaired electron over the Tr-orbital system in each case. 

Allylic and benzylic cations have delocalized electrons, so they are more stable than similarly substituted carbocations with localized electrons. An allylic cation is a carbocation with the positive charge on an allylic carbon an allylic carbon is a carbon adjacent to an sp carbon of an alkene. A benzylic cation is a carbocation with the positive charge on a benzylic carbon a benzylic carbon is a carbon adjacent to an sp carbon of a benzene ring. 

Because the allyl and benzyl cations have delocalized electrons, they are more stable than other primary carbocations. (Indeed, they have about the same stability as secondary alkyl carbocations.) We can add the benzyl and allyl cations to the group of carbocations whose relative stabilities were shown in Sections 4.2 and 6.4. 

The benzylic cation rearranges to form cycloheptatrienyl cation, which is even more stable. Note that the cycloheptatrienyl carbocation is aromatic 6 7T electrons) and has the positive charge delocalized around the entire ring. 

As shown in the following mechanism, reaction is initiated by heterolytic cleavage of the carbon-chlorine bond to form a 2° carbocation, which rearranges to a considerably more stable 3° carbocation by shift of a hydrogen with its pair of electrons (a hydride ion) from the adjacent benzylic carbon. Note that the rearranged carbocation is not only tertiary (hyperconjugation stabilization) but also benzylic (stabilization by resonance delocalization). 

Now we will look at allylic and benzylic cations. These are carbocations that have delocalized electrons and are therefore more stable than similar carbocations with localized electrons. 

---