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If Resonance can stabilize both carbo cation and anion then why can't hyperconjugation stabilize carbanion ? Since both Resonance and Hyperconjugation are pretty much the same, i.e both involve delocalisation or diffusion of electrons( at least in model) ?

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  • $\begingroup$ Unfavourable geometry. $\endgroup$
    – Mithoron
    Dec 3, 2019 at 19:15

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Hyperconjugation involves the electrons from an adjacent bond to "donate" electrons to an empty p orbital. Now if we consider a filled p orbital, or a carbo-anion, then there will be a repulsive force according to Coulomb's law. This will destabilize the molecule in question. However, resonance involves the delocalization of electrons in a molecule; whether this stabilizes the molecule depends on what exactly is going on.

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In carbanion the carbon carrying negative charge contains one $\mathrm{sp^3}$ hybrid orbital which contains a pair of electrons and therefore there is no possibility that it will undergo bonding interaction with the neighbouring $\ce{C-H}$ bond, due to repulsion between the electrons.

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