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I have to an extent understood hyperconjugation but I don't understand why the donation can only occur from a $\mathrm{sp^3}$-hybridised orbital bonded with hydrogen? What about other bonding partners?

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  • $\begingroup$ It doesn't have to be $\mathrm{sp^3}$ hybridised (the issue with $\mathrm{sp^2}$ and $\mathrm{sp}$ is not so much that they're not $\mathrm{sp^3}$, but rather because of the bond angles and restricted rotation in unsaturated systems which forbid correct overlap). It also doesn't have to be hydrogen, but hydrogen is the most common example. $\endgroup$
    – orthocresol
    Feb 1 at 15:26
  • $\begingroup$ Hyperconjugation do occur with C-F bond as well where the fluorine gains electron instead of losing(as in the case of hydrogen) $\endgroup$
    – Govind
    Feb 1 at 16:29
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No.

Hyperconjugation is the donation of electrons from a one orbital to another, where at least one of those orbitals is a bonding orbital, and there is a net overlap between the orbitals.

A typical first example taught of hyperconjugation is that which occurs in the conformations of simple alkanes (e.g. ethane). When ethane is in the staggered conformation, electrons from the $\sigma_\ce{CH}$ bonding orbital are donated to the properly aligned $\sigma_\ce{CH}^*$ antibonding orbital of the adjacent carbon atom. The interaction of these two orbitals, the $\sigma_\ce{CH} \to \sigma_\ce{CH}^*$ lowers the energy of those electrons, and thus lowers the energy of the molecule.

Next up is to look at a longer alkane chain (e.g. butane), which will have $\sigma_\ce{CH} \to \sigma_\ce{CH}^*$ interactions like in ethane, but will also have additional interactions involving the $\sigma_\ce{CC}$ and $\sigma_\ce{CC}^*$ bonding & antibonding orbitals. Depending on which conformation of butane you are considering, it is possible to have $\sigma_\ce{CC} \to \sigma_\ce{CH}^*$, $\sigma_\ce{CC} \to \sigma_\ce{CC}^*$, and $\sigma_\ce{CH} \to \sigma_\ce{CC}^*$ interactions.

Continuing in a more general sense, these interactions are not limited to $\sigma$ bonding orbitals. In carbocations, which in principle have an empty p orbital ($\ce{C}_\mathrm{2p}$) on the electron-deficient carbon, there can be hyperconjugation in the form of $\sigma \to \ce{C}_\mathrm{2p}$. (Note that I didn't specify the atoms involved in the $\sigma$ bond; as long as the atoms form a stable $\sigma$ bond, this hyperconjugation interaction can occur, though how stabilizing it is will depend on the size of the orbitals involved.)

There are a variety of hyperconjugation interactions that can occur:

  • $\sigma \to \sigma^*$
  • $\sigma \to \pi^*$
  • $\sigma \to$ empty hybridized orbital
  • hybridized orbital $\to \sigma^*$
  • $\pi \to \sigma^*$

For example, in protonated formamide ($\ce{H-C(=O)-NH3+}$), there is hyperconjugation in the form of $\pi_\ce{CO} \to \sigma_\ce{NH}^*$ and $\sigma_\ce{NH} \to \pi_\ce{CO}^*$, as well as $\ce{O}_\mathrm{2p} \to \sigma_\ce{CN}^*$.

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    $\begingroup$ Nice answer! I might add that the $n \to \sigma^*$ and $\pi \to \sigma^*$ interactions are often called "negative hyperconjugation", cf. the IUPAC Gold Book. $\endgroup$
    – orthocresol
    Feb 1 at 21:35
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    $\begingroup$ Interesting, I hadn't heard of that naming before. (Also, thanks for cleaning up the post.) $\endgroup$
    – A will O
    Feb 1 at 21:49

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