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Recently I read about Hyperconjugation. But i have a doubt. enter image description here

What is the need of hyperconjugation in this structure when there is charge separation resulting in lesser stable structure.

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  • $\begingroup$ delocalization of electrons takes place through overlap between sigma bond orbital and pi-bond orbital or p- orbitals which dominates unstability due to charge seperation and causes overall stability $\endgroup$ – Heisenberg Mar 19 '15 at 7:16
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What is the need of hyper conjugation in this structure when there is charge separation resulting in lesser stable structure.

I think we'd all agree that the charge separated structure you've drawn is a resonance structure. Charge has been separated so it may well be higher in energy than the first (neutral) resonance structure. Nonetheless, to whatever extent the charge separated resonance structure contributes to the description of propene - even if it is just a small amount - it will stabilize propene.

Since there are 3 hydrogens on the methyl group you can draw three identical resonance structures, this will increase the importance of the structure. Note too the charge separated resonance structures involve an allyl carbanion (resonance stabilized) and a proton; two very stable structures - this will further increase the weight, or importance, of the hyperconjugated structures.

These hyperconjugated resonance structures are really showing us that an $\ce{sp^3}$ carbon can donate electron density to an $\ce{sp^2}$ hybridized carbon. Electron movement in this direction lowers the energy of the electron because it moves the electron from an orbital that is 25% s-character to an orbital that is 33% s-character. The more s-character in an orbital, the lower the energy of the orbital (an s orbital is lower in energy than a p orbital). As we stabilize the electron, we stabilize the molecule.

This hyperconjugation argument comes in handy. It can be used to explain

  • why double bonds with more alkyl groups are more stable than double bonds with fewer alkyl groups (ref1, ref2,)
  • why carbocation stability follows the order: tertiary > secondary > primary (ref1, ref2)
  • why alkyl groups are ortho-para directing in electrophilic aromatic substitution (ref)
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