Resonance benzylic carbocations

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.

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of S l reactivity for alkyl halides and tosylates. 

Benzylic acid rearrangement, 836 Benzylic carbocation, electrostatic potential map of, 377 resonance in, 377 SN1 reaction and, 376-377... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

The results of ab initio calculations provide evidence that Me2NC(S)-[14+] is stabilized by resonance electron donation from the a-thioamide group (A, Scheme 12) and by covalent bridging of sulfur to the benzylic carbon (B, Scheme 12).96 Direct resonance stabilization of the carbocation will increase the barrier to the nucleophile addition reaction, because of the requirement for the relatively large fractional loss of the stabilizing resonance interaction (A, Scheme 12) at the transition state for nucleophile addition to a-substituted benzyl carbocations.8,13,28 91-93 If the solvent adds exclusively to an open carbocation that is the minor species in a mixture of open and closed ions, then... 

The intrinsic barrier for the addition of solvent to an a-alkoxy benzyl carbocation is several kcal mol-1 smaller than that for the corresponding reaction of ring-substituted 1-phenylethyl carbocations. This result is consistent with the conclusion that these nucleophile addition reactions become intrinsically easier as stabilizing resonance electron donation from an a-phenyl group to the cationic center is replaced by electron donation from an a-alkoxy group. 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

Because of its resonance stabilization, the secondary benzylic carbocation is more stable than the secondary carbocation, so the product has the bromine (nucleophile) bonded to the carbon attached to the phenyl group. 

The benzyl carbocation, like the allyl carbocation, has substantial resonance stabilization. The base peak at mlz 91 in the spectrum of butylbenzene (see Figure 15.11) results from a fragmentation that produces a benzylic carbocation as illustrated in the following equation. (We will see in Chapter 16 that the benzylic carbocation rearranges to an isomeric carbocation, which is even more stable.)... 

Three resonance forms for the carbocation of the formula C13H9 are shown below, and more can be drawn. These forms show that the positive charge of the carbocation can be stabilized in the same way as an allylic or benzylic carbocation is stabilized - by overlap with the neighboring n electrons of the ring system. 

The difficulties encountered in using the analysis of substituent effects in solvolyses as a mechanistic probe mostly arise from the mechanistic involvement of the solvent (Shorter, 1978, 1982 Tsuno and Fujio, 1996). Consequently, the behaviour of benzylic carbocations in the gas phase should be the best model for the behaviour of the solvolysis intermediate in solution (Tsuno and Fujio, 1996). The intrinsic substituent effects on the benzylic cation stabilities in the gas phase have also been analysed by equation (2), and they will be compared here with the substituent effects on the benzylic solvolysis reaction. In our opinion, this provides convincing evidence for the concept of varying resonance demand in solvolysis. Finally, we shall analyse the mechanisms of a series of benzylic solvolysis reactions by using the concept of a continuous spectrum of varying resonance demand. 

The ra value significantly increases as the stability of the parent carbocation decreases, while the po value remains rather constant for a series of benzylic carbocations. The Tq values are plotted against the relative alkene basicities G(cc)h+ corresponding to the unsubstituted (parent) carbocations in Fig. 32. It is remarkable that the resonance demand parameters characterizing the stabilities of this series of cations are linearly related to the intrinsic gas-phase stabilities of the parent carbocations. 

These theoretical considerations reveal that the empirical r values are intimately related to the theoretical indices of the structures of benzylic carbocations derived from resonance theory. The coefficient r in the Y-T equation can thus be replaced as a first approximation by a set of theoretical quantities, e.g. increment of bond orders (PAr,a)H as in (35) or sum of charge populations in the aryl ring (2 for the parent carbocations (X = H) as... 

The intermediate benzylic carbocation is stabilized by resonance. Draw the contributing resonance structures. 

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