HOMO/LUMO questions

[schlieren]

I'm very much confused about the MO theory. Let me try to figure out by asking some very naive questions ...
So first of all, hybridization (sp3 sp2 sp) only determines molecular orientations (tetrahedral etc.), and has nothing to do with HOMO/LUMO (bonding, non-bonding, anti-bonding), right?
Next, it seems in homo-diatoms like O2, non-bonding lone pairs occupy 2s and are lower energy than bonding pairs, thus the HOMO is pi(O=O), yet in hetero-diatoms like CO, the non-bonding lone pair is higher than pi bonding and is the HOMO. Why?? When is non-bonding higher than bonding and when not?
I will start will these but have many confusing questions to follow ... thanks a lot in advance

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[NJPreMD]

Traditionally, I have had HOMO/LUMO interactions taught to me using the Diels-Alder reaction as a model to understand the energetics, at both the undergraduate and graduate level.

Understand how to draw the energy levels for butadiene and ethylene. The MO with he lowest energy has no nodes, and the MO with the highest energy has the maximum number of nodes (it's impossible to draw the shaded nodes in this applet, but this could be googled easily enough). MO's are disposed such that the center level is the nonbonding level alpha. If you are able to draw the energy levels correctly and label the HOMO LUMO correctly, you will be able to visualize which HOMO/LUMO interactions are thermally allowed as they will be in phase. (Or this can be determined using the Woodward-Hoffmann rules).

A good test of understanding would be to draw the allyl radical, carbocation, and carbanion. All have 3 MO's but have different number of electrons and therefore have different HOMO/LUMO.

I wouldn't worry about hybridization beyond sp2, as the only case in which you would have to be doing an sp3 hybridized carbon would be in electrocyclic reaction mechanisms where a sigma bond is broken (and controtation/disrotation is generally beyond the scope of an undergraduate organic chemistry course).

 

[BerkReviewTeach]

I'm very much confused about the MO theory. Let me try to figure out by asking some very naive questions ...
So first of all, hybridization (sp3 sp2 sp) only determines molecular orientations (tetrahedral etc.), and has nothing to do with HOMO/LUMO (bonding, non-bonding, anti-bonding), right?
Next, it seems in homo-diatoms like O2, non-bonding lone pairs occupy 2s and are lower energy than bonding pairs, thus the HOMO is pi(O=O), yet in hetero-diatoms like CO, the non-bonding lone pair is higher than pi bonding and is the HOMO. Why?? When is non-bonding higher than bonding and when not?
I will start will these but have many confusing questions to follow ... thanks a lot in advance

It's not tested on the MCAT beyond resonance theory, so you don't really need to worry about it. Know the Diels Alder reaction and you'll be fine.

 

[schlieren]

Traditionally, I have had HOMO/LUMO interactions taught to me using the Diels-Alder reaction as a model to understand the energetics, at both the undergraduate and graduate level.

Understand how to draw the energy levels for butadiene and ethylene. The MO with he lowest energy has no nodes, and the MO with the highest energy has the maximum number of nodes (it's impossible to draw the shaded nodes in this applet, but this could be googled easily enough). MO's are disposed such that the center level is the nonbonding level alpha. If you are able to draw the energy levels correctly and label the HOMO LUMO correctly, you will be able to visualize which HOMO/LUMO interactions are thermally allowed as they will be in phase. (Or this can be determined using the Woodward-Hoffmann rules).

A good test of understanding would be to draw the allyl radical, carbocation, and carbanion. All have 3 MO's but have different number of electrons and therefore have different HOMO/LUMO.

I wouldn't worry about hybridization beyond sp2, as the only case in which you would have to be doing an sp3 hybridized carbon would be in electrocyclic reaction mechanisms where a sigma bond is broken (and controtation/disrotation is generally beyond the scope of an undergraduate organic chemistry course).

Thanks a lot! I will limit my study to those examples you specified. And thanks for the soothing official confirmation from TBR.