# The effect of hyperconjugation on the stability of alkenes with MO theory

Hyperconjugation stabilizes carbocations and that makes sense because electrons are given to the empty p orbital.

But how does it stabilize alkenes? Can molecular orbital theory be used to explain it?

Since you are familiar with how carbocations are stabilized via hyperconjugation I will keep this brief: With alkenes, it really is the same principle at work. Instead of the empty p orbital, you have to consider the empty $\pi^{*}$ orbitals of the $\ce{C=C}$ double bond. They interact with the filled high-lying neighboring $\ce{C-H}$ or $\ce{C-C}$ $\ce{\sigma}$ bonds in $\pi$-bond-like manner

and so from those two fragment orbitals you will get two molecular orbitals: one from the in-phase combination of $\pi^{*} (\ce{C=C})$ with $\sigma (\ce{C-X})$, which will be lower in energy then the fragment orbitals, and one from the out-of-phase combination of $\pi^{*} (\ce{C=C})$ with $\sigma (\ce{C-X})$, which will be higher in energy then the fragment orbitals. Since in this interaction there are only two electrons involved, i.e. the ones in $\sigma (\ce{C-X})$, you get a stabilization from the interaction since those two electrons will now occupy the in-phase MO and have thus lowered their energy. The out-of-phase MO is empty and doesn't contribute to the energy of the molecule.

• The hydrogen in the pictures should only have an s-orbital, but that's just a minor thing and not really important to the underlying concept. More important is that there is also the interaction of $\ce{\pi(C=C)->\sigma^*(C-H)}$. Strictly speaking this is the Valence Bond Interpretation of hyperconjugation. In Molecular Orbital Theory you just mix all orbitals at the same time and use what works best, i.e. gives the lowest energy. – Martin - マーチン Dec 27 '16 at 10:37
• @Philipp May you please cite a reference or book or sth for this ? – Matt Mar 4 '17 at 14:05
• @RaghavSingal A reference (book) for this could be "Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective", by Frank Weinhold and Clark R. Landis. Section 3.4 talks about hyperconjugation in the s- and p-block elements. – Felipe S. S. Schneider Apr 19 '17 at 12:07

When the sp2 hybridized Carbon atom directly attached to sp3 hybridized Carbon atom having at least 1 hydrogen atom(called α hydrogen), this condition is known as hyperconjugation. Stability of any hyperconjugative hydrocarbon depends upon no. of α hydrogen present in compound..

More the no. of α hydrogen, more is the stability of compound

• You make it sound that only CH bonds can undergo hyperconjugation, that's not true. – DSVA Sep 6 '17 at 8:00