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I know that hyperconjugation leads to delocalization of electrons between filled and unfilled orbitals and thus greater stability (that's what most textbooks said).

However, I think that it reduces the bond strength, because by saying that the electrons delocalized would mean that the electron density between two atoms decreases and hence the 'glue' (bond) that hold the two atoms together becomes weaker.

Any misconception here? Because my statement seems to contradict with the fact that hyperconjugation increasing stability. Why reduction in bond strength increase stability? (or does it actually decrease the bond strength?)

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  • $\begingroup$ The little ionic character does not weak any bond..... You don't have to see the sacrifice of a covalent bond as leading to unbound atoms! $\endgroup$ – Alchimista Feb 3 at 9:57
  • $\begingroup$ Think of salts for instance! $\endgroup$ – Alchimista Feb 4 at 9:14
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What you are ignoring is the magnitude of the effect.

While hyperconjugation does decrease the electron density of a localized bond and slightly increases the energy, the overlap with a high-energy, empty orbital is at least very poor. Think that the orbitals are very far from each other and have different symmetries. The total electron density of this bond would remain largely unaffected.

On the other hand, the difference between zero electrons and partial electron density for an empty orbital of high energy is significant, and it increases (to a moderate extent) the stability of, say, a carbocation. Besides, a single orbital often receives electron density by hypercongujation with more than one bond.

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Hyperconjugation effect also improves stability by distributing the positive charge or the radical electron on the involved atoms . Also for alkenes after hyperconjugation effect the pie bond shifts between other atoms thus decreasing the bond lengths of Sigma bonds( involved in the effect) and thereby strengthening their bond. Thus overall it stabilizes the compound.

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This requires molecular orbital theory, which (it seems) is something organic chemistry classes absolutely refuse to teach.

Basically, it goes along these lines:

In the case of CH3+, you only have the 3 sigma bonds (A1’ and E’), meaning you have 6 electrons delocalised along the entire molecule and holding it together.

But in the case of CH2CH3+, the added methyl group can form a pi bond with The Central carbon; this forms a pi bond that stabilises the ion.

I recommend either An Introduction to Molecular Orbitals or Orbital Interactions in Chemistry (which I just bought recently-good book.)

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