[science123]

I don't understand why the first structure is considered non-aromatic instead of anti-aromatic. I was told that the Nitrogen is sp3 so it can't be aromatic but if we draw the resonance structure, then it's sp2 and the contributor has all atoms sp2 in a cyclic manner which should make it anti-aromatic (8 pi electrons).

In case of furan, why isn't the Oxygen considered sp3 this time? Why do we apply different rules for determining hybridization for molecules. Any input would be appreciated.

[macman104]

What about the bottom nitrogen?  Is it sp3 or sp2?

[science123]

What about the bottom nitrogen?  Is it sp3 or sp2?

If I draw another resonance contributor with a double bond between carbon and bottom nitrogen, then it would also be sp2.

[macman104]

Yes, in hindsight, that was a dumb question, as it is a symmetric molecule lol.

Upon further investigation, what is important to take home from this is that while it is true you can characterize the hybridization as sp2 for both nitrogens, antiaromaticity is a destabilizing effect.  So if the system can prevent proper orbital overlap to hinder the antiaromatic resonance, it will.  The most famous example is cyclooctatetraene, it would be classified as antiaromatic, but it adopts a "tub" conformation that prevents the correct overlap.

Just like in the dihydropyrazine, if one of the nitrogens "stays" sp3, it prevents the overlap and the antiaromatic character from being as prominent.  The furan however can be sp2 and become more stabilized by aromatic resonance.

[science123]

Upon further investigation, what is important to take home from this is that while it is true you can characterize the hybridization as sp2 for both nitrogens, antiaromaticity is a destabilizing effect.  So if the system can prevent proper orbital overlap to hinder the antiaromatic resonance, it will. 

What about cycloheptatrienyl anion? It is considered to be anti-aromatic. Why can't the carbon with the lone pair hybridize itself as sp3 instead of sp2 to avoid anti-aromaticity?

[science123]

Anyone?

[sjb]

What other criteria are there for aromaticity?

[science123]

What other criteria are there for aromaticity?

All atoms sp2, cyclic and planar as I know. Is there anything else?

[tarkolaszlo]

Huckel model of aromaticity is obsolete.
See TOPAZ algorithm, ARKIVOC, 2008, XI, p. 24.

1,4-dimethyl-1,4-dihydropyrazine cycle IS NOT aromatic. There are "single" and "double" chemical bonds only, not "aromatic" bonds. Therefore, within the dihydropyrazine cycle is "discontinuous conjugation".

[tarkolaszlo]

If you wish to compute aromaticity for any thinkable chemical structure I can send freeware, by E-mail, DESCRIPT software (see again ARKIVOC, 2008, XI, p. 24)

[macman104]

Upon further investigation, what is important to take home from this is that while it is true you can characterize the hybridization as sp2 for both nitrogens, antiaromaticity is a destabilizing effect.  So if the system can prevent proper orbital overlap to hinder the antiaromatic resonance, it will.  

What about cycloheptatrienyl anion? It is considered to be anti-aromatic. Why can't the carbon with the lone pair hybridize itself as sp3 instead of sp2 to avoid anti-aromaticity?

Sorry, I should have been more specific, the nitrogens are likely some hybrid between sp2 and sp3, however, the system distorts itselfs so that the p orbitals do not overlap correctly for complete anti-aromatic delocalization.  Cyclooctatetraene should be antiaromatic, all sp2, cyclic, however, it distorts so that it does not become planar.

When you get to the antiaromatic compounds it is not always clearcut.  By nature, they are highly unstable, and will do just about anything to not be antiaromatic, so weird things can happen.  By applying your rules, you would initially guess that cyclooctatetraene is antiaromatic, there is no way except experimentally that you would know it forms the "tub" shape.  I am a little surprised that cycloheptatriene anion cannot twist enough to get out of antiaromaticity, but maybe that extra carbon from 7 to 8 allows for the correct flexibility.

If anyone has a more satisfying answer, please feel free to chime in, as it seems a little "hand-wavy" to me

[science123]

My professor said when asked that 1,4-dimethyl-1,4-dihydropyrazine is a neutral molecule and it is unlikely to go from neutral to charged (high energy) resonance structures. If there isn't any resonance, then there is no way for the nitrogens to be sp2 and the molecule has to be non-aromatic. Furan which is neutral has charged resonance structures because they are aromatic making all the atoms sp2.

In case of cycloheptatrienyl anion, it has resonance structures because it is already negatively charged and that negative charge needs to be delocalized. So, all the carbons has to be sp2 and it can't escape anti-aromaticity. I think that the stability gained by resonance for negatively charged molecules (like cylcoheptatrienyl anion) outweighs the instability caused by anti-aromaticity but not really sure. I don't know if i can apply this to different neutral or anions.