Orgo - hund's rule for hybrid orbitals?

[ysurge19]

When you have sp3 hybridization, you fill one electron in each orbital right, thus you have 4 unpaired orbitals for bonding.

But with sp2 hybridization, you have an empty p orbital that is higher in Energy than the 3 sp2 hybrid orbitals...

so now to my question: when such an atom is in a sp2 bond, is the p orbital always unpaired?

Isn't it true that (at least one form of) resonance requires adjacent p orbitals to be unpaired?
Yet how can a double-bonded atom be able to have this resonance if that means it's p-orbital must be overlapping with the adjacent atoms p-orbital (in order to be a double bond...) thus making it actually unavailable for resonance?

For example, carbon of benzene are all in a conjugated pi system (all Carbons are sp2) meaning they all have an unpaired p orbital? so how can there be double bonds between the carbons if that would imply that the p-orbitals are not available for resonance?

Or is it that aromatic compounds double bonds are not truly double bonds thus the p orbital is actually unpaired?

..thanks for your time

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[DAT Destroyer]

When you have sp3 hybridization, you fill one electron in each orbital right, thus you have 4 unpaired orbitals for bonding.

But with sp2 hybridization, you have an empty p orbital that is higher in Energy than the 3 sp2 hybrid orbitals...

View attachment 218495

so now to my question: when such an atom is in a sp2 bond, is the p orbital always unpaired?

Isn't it true that (at least one form of) resonance requires adjacent p orbitals to be unpaired?
Yet how can a double-bonded atom be able to have this resonance if that means it's p-orbital must be overlapping with the adjacent atoms p-orbital (in order to be a double bond...) thus making it actually unavailable for resonance?

For example, carbon of benzene are all in a conjugated pi system (all Carbons are sp2) meaning they all have an unpaired p orbital? so how can there be double bonds between the carbons if that would imply that the p-orbitals are not available for resonance?

Or is it that aromatic compounds double bonds are not truly double bonds thus the p orbital is actually unpaired?

..thanks for your time

You have asked a great question. Let's start with a double bond. A double bond has two components when you discuss elementary orbital theory. The sigma bond and the pi bond. In actuality.....many many orbitals are involved, but I will keep it simple as the undergrad texts do......The double bond is said to be sp2,,,,,thus a single orbital namely a p orbital is left over..UNHYBRIDIZED.......this left over orbital is able to overlap with another adjacent orbital that is parallel to form the pi bond. In a triple bond, we see the sp hybrid model. There is one sigma bond connecting the two carbons,,,,and we have 2 empty p orbitals that are UNHYBRIDIZED........they overlap perpendicularly or orthogonally as we like to say in organic chemistry parlance,,,,,,to for the form the 2 pi bonds. This constitutes the bonding orbitals.......In actuality, this is only a rough approximation that will guide you. In benzene,,,,,we have 6 carbons,,,,,thus 6 molecular orbitals can be created, 3 are bonding, and 3 are antibonding. Regardless, they are all made by unhybridized p orbitals ! For a nice look at pictures of this, the Klein book illustrates this best !!!! In Advanced Organic Chemistry, the work of Ian Fleming is my favorite, but not for the neophyte. Have a look at the Klein orbital pictures, and if you need further clarity, feel free to ask me.

Hope this helps.

Dr. Jim Romano

 

[ysurge19]

.....The double bond is said to be sp2,,,,,

Thank you for the response Dr. Romano.

Is it true that the double bond can only result from p-orbital overlap thus can't be referred to as sp2?

Also for benzene you said the 6 carbons have 6 bonds, 3 are antibonding and 3 are bonding.
I was under the impression that each bond itself possesses a bonding molecular orbital and an antibonding molecular orbital (making 6 of each, one for every bond).