[Winga]

Why hyperconjugation can stabilize alkenes?

The e- pair from C-H sigma (bonding) orbital donates to c=c pi* (anti-bonding) orbital. This will weaken the C-H bond and C=C bond according to MO theory.
So, how can it stabilizes the whole system?

[movies]

More substituted olefins are certainly more electron rich, so they will react faster with electrophilic reagents than their unsubstituted counterparts.  They tend to react slower in other reactions, however, because they are more sterically hindered.

[Ashutosh]

i think that concept of hyperconjugation is used to only explain observed fact i.e.stability.it is observed that more the number of hyperconjugative structure more is the stabilty.it is also known as no bond resonance.remember,stability can be explained by this phenomenon,there can be other reason also.confirm it.

[Winga]

I find that (from notes) the e- pair donation from C-H / C-C sigma orbital to C=C pi* orbital will stabilize the pi* orbital.

Is this explanation acceptable?

[hmx9123]

Bear in mind that an alkene happens to be electron deficient.  The argument of hyperconjugation is that the sigma bond of one substituent on the double bond donates to the pi* orbital as you have mentioned.  That allows the electron deficient carbon to 'share' in some of the electron density of the substituent.  It's a poor resonance structure to be sure, but it's another resonance structure, and, as someone mentioned, is an explanation of an observed experiment.  It is a little fishy.  Perhaps this is a bit oversimplified.

Also, not all more highly substituted alkenes are more electron rich.  It depends on the groups.  Take a look at tetrafluoroethylene, for instance.  That's certainly less electron rich than ethylene.  This is, of course, speaking of the double bonds, not about the number of electrons in the total molecule.  Yes, TFE has more electons than ethylene, but the double bond in TFE is much more electrophilic.

[Winga]

Bear in mind that an alkene happens to be electron deficient.

Which explanation is better to describe that an alkene is electron deficient?

1. The pi electrons are not concentrated between the nuclei?

2. The two C=C carbons are sp2 hybridized, so, they are more electronegative than the sp3 hybridized carbons that give higher tendency to draw electrons?

3. How about using s & p characters approach? More s-character, then...? I don't know how to explain if using this approach.

[hmx9123]

An alkene isn't sp3 hybridized, which is what you're looking for in terms of electron 'happiness'.  Any unsaturated system is deficient of electrons because it's not saturated.  It has to share electrons with a neighboring atom to make up the octet.

[Winga]

According to a source, the hyperconjugation stablizes the alkenes by donating the C-H/C-C sigma orbital e- density to pi* orbital of C=C.

It stated that the sigma orbital interacts/overlaps with the pi* orbital to form 2 new MOs, and the pair e- of sigma orbital donates to the new bonding MO which is the lowest in energy.

Is it a good explanation? (I think it is.)

[Winga]

Here.

[Winga]

Is this answer which using MO theory correct?

[Winga]

Any comments about it?

[movies]

Looks about right to me.

There might be a small contribution from the other lobe of the filled sp3 orbital too.  You might also want to consider what the overlap between the sp3 antibonding orbital with the pi-antibonding orbital would look like.