Carbocations by hyperconjugation

In molecular orbital terms, alkyl groups can stabilize a carbocation by hyperconjugation. This is the overlap of the filled a orbitals of the C—H or C—C bonds adjacent to the carbocation with an empty p orbital on the positively charged carbon atom. As a result, the positive charge is delocalized onto more than one atom, and thus increases the stability of the system. The more alkyl groups there are attached to the carbocation, the more a bonds there are for hyperconjugation, and the more stable is the carbocation. 

The stabilisation of a carbocation by hyperconjugation by a P-C-Sn bond is sometimes referred to simply as the P-tin effect. It is apparent in the abstraction of hydride by Ph3C+ BF4 from alkyltin compounds.19 Tetramethyltin is unreactive, but ethyltin compounds undergo elimination. 

Stabilization of a carbocation by hyperconjugation The electrons of an adjacent C—H bond in the ethyl cation spread into the empty p orbital. Hyperconjugation cannot occur in a methyl cation. 

No. Recall that stabilization of carbocations by hyperconjugation involves -IT overlap between a bonding hybrid orbital and an empty orbital on carbon (Section 7-5). There aren t any empty p orbitals on oxygen in alkyloxonium ions therefore such overlap is not possible. 

Scheme 2.5 (a) Stabilization of carbocation by hyperconjugation with an adjacent methyl... 

Only electrons in bonds that are ]8 to the positively charged carbon can stabilize a carbocation by hyperconjugation. Moreover, it doesn t matter whether H or another... 

The possibility that strained-ring systems other than cyclopropyl may stabilize carbocations by hyperconjugation is a matter of considerable interest. In 1969, Jensen and Smart reported that the partial rate factors for benzoylation at the para positions of norbornyl benzenes were greater than for simple alkyl benzenes and that, in contrast to the latter, the compound with the tertiary substituent 1-phenylnorbornane reacted faster than those with a secondary substituent, 7-phenylnorbornane and exo-and ndo-2-phenylnorbornane. It was also found that exo-2-phenyl-norbornane reacted faster than ndo-2-phenylnorbornane and isopropylbenzene (relative reactivities, 2.89 1.85 1). These results were attributed to enhanced C—C hyperconjugation in the phenylnorbornanes. There was already evidence from —H coupling constants that the C—H bonds... 

We have seen that alkyl substituents (such as CH3) stabilize carbocations by hyperconjugation— that is, by donating electrons to an empty p orbital (Section 6.2). 

This reanangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary car bocation are shown in structure (a), those of the transition state for reanangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged car bon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... 

The stability order can be explained by hyperconjugation and by the field effect. In the hyperconjugation explanation, we compare a primary carbocation with a tertiary. It is seen that many more canonical forms are possible for the latter ... 

As with carbocations, the stability order of free radicals is tertiary > secondary > primary, explainable by hyperconjugation, analogous to that in carbocations... 

A more complete discussion of the mechanism of addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are involved is considered there. Even when a carbocation is not involved, the regioselectivity of electrophilic addition is the result of attack of the electrophile at the more electron-rich carbon of the double bond. Alkyl substituents increase the electron density of the terminal carbon by hyperconjugation (see Part A, Section 1.1.8). 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

This corresponds to an isotope effect of approximately 3.5% per deuterium. In comparison, the secondary /3-deuterium KIEs in SN1 reactions are all normal and range from 5% to 15% per deuterium. Because the normal KIEs in SN1 reactions result from the weakening of the C,—L bond by a hyperconjugative interaction with the incipient carbocation in the transition state, the authors concluded that hyperconjugative interactions are present also in the transition state for the insertion reaction. The normal secondary /3-deuterium KIE observed for the insertion reaction is consistent with the dipolar three-centre transition state structure [15] proposed by Seyferth et al. (1970a,b) because the partial positive charge on the a-carbon is stabilized by hyperconjugation. 

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. 

In general, SnI rates at an allylic substrate are increased by any substituent in the 1 or 3 position that can stabilize the carbocation by resonance or hyperconjugation.262 Among these are alkyl, aryl, and halo groups. 

These data show a decrease in the extent of /J-silyl stabilization with successive methyl substitution. The methyl (and phenyl) substituents stabilize the carbocation by polarization and inductive effects, resulting in a delocalization of positive charge away from the carbocation, and therefore a reduction in hyperconjugative interaction with the fi-substituent bond. 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

Provided that the silicon-carbon bond can be coplanar with the vacant p orbital, the /J-silyl substituted carbocation should be stabilized by hyperconjugation, and this has been demonstrated by Kresge and coworkers47,49. 

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