Carbocation resonance effects

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... 

In contrast, polar and resonance effects must be separated in order to analyze the data for a-substituted arylolefins [ArC(R)=CHR with R H]. Their bromination involves open carbocation intermediates only. Resonance effects cannot be fully developed at the transition states, since the aromatic ring is not in the same plane as that of the developing carbocation, because of steric constraints. Accordingly, application of (33) gives pT < pn. Attenuation of resonance arises mainly from stereochemical factors, at least in the monosubstituted 1,1-diphenylethylene [20] and a-methylstilbene [21] series the pr/pn ratios can be related to the dihedral angle between the substituted phenyl ring and the plane of the ethylenic bond. 

Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the Inductive effect for the attack at ortho- and para-positions and hence makes the deactivation less for ortho- and para-attack. Reactivity Is thus controlled by the stronger Inductive effect and orientation Is controlled by resonance effect. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Problem 16.46 shows the mechanism of the addition of HBr to 1-phenylpropene and shows how the aromatic ring stabilizes the carbocation intermediate. For the methoxyl-substituted styrene, an additional resonance form can be drawn in which the cation is stabilized by the electron-donating resonance effect of the oxygen atom. For the nitro-substituted styrene, the cation is destabilized by the electron-withdrawing effect of the nitro... 

As mentioned in the previous section, localization of the unpaired electron of the cation radicals is crucial for accomplishing a direct carbon-carbon bond formation using the radical cations of enamines. This method, however, has a severe limitation because cation radicals having such a resonance effect are scarce. To develop a general method for generating reactive carbon species such as carbocations or radicals from cation radicals, we have to find another process. 

Resonance effect further stabilizes the carbocations when present. By resonance the positive charge on the central carbon atom gets dispersed over other carbon atoms and this renders stability to the carbocation. The more the canonical (resonating) structures for a carbocation, the more stable it will be. For example, benzyl and allyl carbocations are very stable because of resonance. 

The principles of inductive effects and resonance effects, first introduced in Section 18.6, can now be used to predict carbocation stability. 

Resonance effects. Conjugation with a double bond increases the stability of a carbocation. Thus, allylic and benzylic cations are more stable than their saturated counterparts. (For example, see Problem 1.4.c.) Heteroatoms with unshared electron pairs, e.g., oxygen, nitrogen, or halogen, can also provide resonance stabilization for cationic centers, as in the following examples ... 

The effect of fluorine, chlorine, or bromine as a substituent is unique in that the ring is deactivated, but the entering electrophile is directed to the ortho and para positions. This can be explained by an unusual competition between resonance and inductive effects. In the starting material, halogen-substituted benzenes are deactivated more strongly by the inductive effect than they are activated by the resonance effect. However, in the intermediate carbocation, halogens stabilize the positive charge by resonance more than they destabilize it by the inductive effect. 

Meta-directing deactivators act through a combination of inductive and resonance effects that reinforce each other. Inductively, both ortho and para intermediates are destabilized because a resonance form places the positive charge of the carbocation intermediate directly on the ring carbon atom that bears the deactivating group (Figure 16.17). At the same time, resonance electron withdrawal is also felt at the ortho and para positions. Reaction with an electrophile therefore occurs at the meta position. 

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