Hyperconjugation stabilizing interaction between

The stability order of alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tr bond and adjacent C-H a bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom C=C—< -H grouping, as shown in Figure 6.6. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

Figure 6.6 Hyperconjugation is a stabilizing interaction between an unfilled w orbital and a neighboring filled C-H <7 bond on a substituent. The more substituents there are, the greater the stabilization of the alkene.

Hyperconjugation is a stabilizing interaction between an unfilled 77 orbital and a neighboring filled C-H or bond orbital. 

Hyperconjugation is a stabilizing interaction between a C-H bond and an adjacent a-bond. For example, propene may be regarded as a resonance hybrid of two canonical forms ... 

Hyperconjugation, discussed in Section 7.5 in connection with the stability of substituted alkenes, is the stabilizing interaction between a p orbital and C-H cr bonds on neighboring carbons that are roughly parallel to the p orbital (Figure 7.9). The more alkyl groups there are on the carhocation, the more stable the carhocation. 

FIGURE 7.9 In the ethyl carhocation, CH3CH2", there is a stabilizing interaction between the carhocation p orbital and adjacent C-H methyl substituent, as indicated by this molecular orbital. The more substituents there are, the greater the stabilization of the cation. Only the C-H bonds that are roughly parallel to the neighboring p orbital are oriented properly to take part in hyperconjugation. 

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

Schaefer and coworkers20 have used long-range NMR coupling constants to investigate rotational barriers about the C(sp2)—C(sp3) bonds in benzyl compounds. The barrier for benzylsilane was found to be 1.77 kcalmol-1, compared to 1.2 kcalmol-1 for ethylbenzene. The increased barrier for benzylsilane is attributed to increased stabilization of the stable conformer, in which the C—Si bond lies in a plane perpendicular to the benzene plane, by a hyperconjugative interaction between the C—Si bond and the jr-system. 

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