Orbital overlap hyperconjugation

Hyperconjugation in the above case may be regarded as an overlap of the ct orbital of the C—H bond and the n orbital of the C—C bond, analogous to the n-K orbital overlap previously considered. As might be expected, those who reject the idea of resonance in butadiene (p. 37) believe it even less likely when it involves nobond structures. 

Hyperconjugation. An increase in the number of alkyl substituents increases the stability of the carbocation due to orbital overlap between the... 

However, there is evidence against /-orbital overlap and the stabilizing effects have been attributed to other causes.In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and negative hyperconjugation play a minor role, if any. An ot silicon atom also stabilizes carbanions. ... 

Radicals are well known to attack hydrogen atoms in the order allylic faster than tertiary, faster than secondary, faster than primary. The more neighbouring groups a C—H bond has, the more overlap (hyperconjugation) can take place. Since such overlap is between filled orbitals and filled orbitals, the effect is to raise the energy of the HOMO. This effect therefore puts the energy of the HOMOs of the C—H bonds in the same order as their ease of abstraction. 

Figure 4.5 Carbocation resonance forms and the pi-type orbital overlap that those resonance forms represent. Top Resonance with a lone pair is best. Middle Resonance with a pi bond is next. Bottom sigma bond hyperconjugation is poorest the H, pi bond resonance form is minor.

This stabilization has been shown to be primarily a delocalization effect by demonstrating a strong dependence on the dihedral angle between the Si—C bond and the empty p orbital of the carbocation. Kinetic rate enhancements of about 1012, 0, and 104 were found for compounds with angles of 180°, 90°, and 0°, respectively. This is consistent with a hyperconjugation mechanism and with analyses of orbital overlap that show considerably stronger overlap for the anti (180°) than syn (0°) orientations. ... 

Hyperconjt doii (Sections 4.8B and 6.1 IB) Electron delocalization (via orbital overlap) from a filled bonding orbital to an adjacent unfilled orbital. Hyperconjugation generally has a stabilizing effect. 

We have just seen that carbocations are stabilized by delocalization through the overlap of filled and empty orbitals. One example of this effect is fairly straightforward a lone pair of electrons on an adjacent atom interacts with an empty orbital. Less obvious is the last phenomenon, the overlap of filled O orbitals of carbon—hydrogen bonds with empty orbitals, called hyperconjugation. Now we proceed to other factors influencing the stability of carbocations. 

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