[science123]

I am confused if the first two are anti-aromatic or non-aromatic. There are 4 and 8 pi electrons respectively if we consider the carbanion to be sp2 (due to resonance) which will lead to anti-aromaticity. The problem is that my instructor told us that anti-aromatic compounds would be unstable and they will try to escape anti-aromaticity by changing the shape. I was wondering if that carbion can stay sp3 instead of sp2 to avoid anti-aromaticity but then it has to sacrifice resonance. Resonance vs aromaticity..which one wins ?

For 3rd - would the oxygen be sp3 or sp2 due to resonance? Anti-aromatic or non-aromatic ?

For 4th - There are 6 pi electrons in the resonance contributor making it aromatic. Would the hybrid considered as aromatic then?

[pacifyer]

1 & 2) Antiaromatic anions (4n electrons)

3) Non aromatic (sp3 O)

4) Aromatic: think on the resonance form with C-O single bond (in the C=O), and positively charged ring...

[science123]

1 & 2) Antiaromatic anions (4n electrons)

That didn't quite answer my question. Why can't the carbanion change shape and hybridize itself as sp3 instead of sp2 to break anti-aromaticity thus more stability? Is resonance more important here?

[Arctic-Nation]

I'm not quite sure how it works with the cyclopropenyl anion, but as it has as good as none conformational flexibility, I suspect there are two opposing forces at work: hybridization to sp3 to get rid of the anti-aromaticity and reduce bond strain, and hybridization to sp2 to stabilize the carbanion. No matter what happens, you're still stuck with something that cannot possibly be stable in any way.

As for the cycloheptatrienyl anion, a quick google search revealed that its pKa is about 36, making it almost as strong an acid as 1,4-pentadiene. This indicates that the anion is most certainly not flat and not fully conjugated. While it is isoelectronic with cyclooctatetraene, its pKa value makes me suspect that it is partially conjugated and does not share COT's tub-shaped conformation. Beware, however, this is my conjecture.

However, to reiterate your question, rehybridization of a carbanion from sp2 to sp3 imparts a significant energetic penalty due to the increased p character of the orbital involved, and unless some other stabilization mechanism is at work, it is highly unlikely to happen.