Hyperconjugation the systems of delocalized electrons discussed so far involved only the interactions of electrons. The delocalization of π electrons is also possible with σ electrons. For compounds which have a CH, a CH2, or a CH3, group attached to a doubly bonded carbon atom the following types of contributing structures can be written:
Bond Resonance or Hyperconjugation.
In the canonical form (2) no bond is written between hydrogen and a carbon atom. This type of resonance is called no bond resonance or hyperconjugation. In the canonical form (2) no bond is written between hydrogen and a carbon atom. This type of resonance is called no bond resonance or hyperconjugation. It does not mean that there is any molecule which really has no bond between two atoms. What it means is that in the resonance hybrid the C – H bond under consideration is weaker than a normal C- H bond or it has character between a single and a no bond. Also due to the contribution of structure (2) the bond between C2 and C3, has acquired some double-bond character and consequently is expected to be shorter than an ordinary C-C single bond. Other examples of hyperconjugation are as follows:
In (a) the hyperconjugation shows why toluene has a dipole moment of 0.37 D.
The observed C – C bond length (between C2 and C3) in (b) is 1.47 Å as against 1.54 Å of ordinary C – C bonds. Similarly, bond lengths are different in the following structures due to the presence of hyperconjugation.
Since the resonance stability of the hybrid is greater if more contributing structures are possible, a tertiary carbocation, for example (d) above, is more stable than a secondary carbocation (e) which is more stable than a primary carbocation (f).
In terms of the molecular orbital theory the orbital of the C – H bond has some overlap with the p orbitals of the adjacent unsaturated atoms. This overlap is not as important as that between p orbitals only. The resonance stabilization due to hyperconjugation is significant but not of the same magnitude as due to normal resonance.