anti aromatic vs. non-aromatic

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I still don't understand what the difference between the anti and non aromatic are. anti-aromatic is 4n? but there are situations where 4n can be non-aromatic as well. Someone please help with this, the other posts did not help much.

Thanks

think of a non aromatic ring as one that violates any of the aromaticity rules. it may have an sp3 carbon. it is a lot less stable than the ring that is aromatic.

anti-aromatic is even less stable than non-aromatic. it follows all the aromaticity rules, but has an even # of pi electron pairs, not an odd number.

1. must have a ring
2. every atom must have a p orbital. otherwise --> nonaromatic
3. ring must be flat. otherwise --> nonaromatic
4. 4n+2. if 4n=antiaromatic

If a 4n structure can conform to a nonplanar configuration, it is nonaromatic. I believe the rule of thumb is that any ring with over 8 atoms can conform to nonplanar.

Am I missing anything?

Originally Posted by condoleezarice

1. must have a ring
2. every atom must have a p orbital. otherwise --> nonaromatic
3. ring must be flat. otherwise --> nonaromatic
4. 4n+2. if 4n=antiaromatic

If a 4n structure can conform to a nonplanar configuration, it is nonaromatic. I believe the rule of thumb is that any ring with over 8 atoms can conform to nonplanar.

Am I missing anything?

if its not a ring, its nonaromatic as well

Originally Posted by Contach

if its not a ring, its nonaromatic as well

lol, I think that goes without saying

Originally Posted by condoleezarice

1. must have a ring
2. every atom must have a p orbital. otherwise --> nonaromatic
3. ring must be flat. otherwise --> nonaromatic
4. 4n+2. if 4n=antiaromatic

If a 4n structure can conform to a nonplanar configuration, it is nonaromatic. I believe the rule of thumb is that any ring with over 8 atoms can conform to nonplanar.

Am I missing anything?

How do you know when something is planar or not? I never figured that out..

Also when its sp3 its non-aromatic but I swear sometimes I see an sp3 but its ignored. Like -NH- -O- and -S-

And whats the difference between the N and O in -NR- and -OH- as compared to -NH- and -O-

Originally Posted by yakuza

How do you know when something is planar or not? I never figured that out..

Also when its sp3 its non-aromatic but I swear sometimes I see an sp3 but its ignored. Like -NH- -O- and -S-

And whats the difference between the N and O in -NR- and -OH- as compared to -NH- and -O-

The ring will always conform to the most stable conformation. So, if it can become aromatic by becoming flat, it will. If an anti-aromatic ring can become non aromatic by conforming to nonplanar, it will.

The NH, O, and S you refer to are probably pyrrole, furan, and thiphene. In furan, for example, the carbon atoms only contribute 4 electrons to the pi structure, which is very unstable. One lone pair on the oxygen will conform to become part of the pi system in order to form an aromatic system of 6 electrons. Thus, the O is actually sp2 hybridized. I'm not sure if I explained it well. It would be good to look up heterocyclic aromatic compounds on google.

you explained it correctly. the electrons on those N, O, and S atoms would much rather be in sp2 so as to be aromatic, greatly stabilizing the molecule.

one definition of anti-aromatic i believe is when there are 4 or 8 pi electrons in the ring. even if it is fully conjugated.

Originally Posted by condoleezarice

1. must have a ring
2. every atom must have a p orbital. otherwise --> nonaromatic
3. ring must be flat. otherwise --> nonaromatic
4. 4n+2. if 4n=antiaromatic
(not ring-nonaromatic)

If a 4n structure can conform to a nonplanar configuration, it is nonaromatic. I believe the rule of thumb is that any ring with over 8 atoms can conform to nonplanar.

Am I missing anything?

needed this but not sure I understand: why is cyclooctane non-planar?

I'm not 100% sure, but i think it's bc cyclooctane will form non planar conformations bc of the ring strain.

I was curious to know when to know a cyclic anion's electrons will be a part of the pi system? Chad drew out resonance structures once and showed how it can participate, however I've gotten a lot of Destroyer questions wrong regarding aromaticity bc I keep thinking the extra electrons are actually participating, thus giving them 4n + 2 when in according to the solutions its not.

Tx is right, the huge number of carbons make them greater than 120. Sp2 for all carbons. Since it does not gain stability by becoming planar, not satisfy 4n+2. so better off becoming nonplanar

Originally Posted by SaintJude

needed this but not sure I understand: why is cyclooctane non-planar?

Cyclooctatetraene is floppy enough to be able to bend out of the plane. If it's a planar structure it has an energetic penalty ("antiaromaticity") for being so. If you make a model you'll see why it's floppy.

Cyclobutadiene and cyclopentadienyl cation (two examples of antiaromatic compounds) are not floppy enough to bend out of plane.

Originally Posted by TXpredent

I'm not 100% sure, but i think it's bc cyclooctane will form non planar conformations bc of the ring strain.

I was curious to know when to know a cyclic anion's electrons will be a part of the pi system? Chad drew out resonance structures once and showed how it can participate, however I've gotten a lot of Destroyer questions wrong regarding aromaticity bc I keep thinking the extra electrons are actually participating, thus giving them 4n + 2 when in according to the solutions its not.

Yes, a cyclic anion's electrons can be part of the pi system so long as the atom with the lone pair is not also part of a pi bond. Only one p orbital per atom can contribute to the pi system. This is why, for instance, we ignore pyridine's lone pair for aromaticity purposes [the N is part of a pi bond] , but we count the lone pair for pyrrole [it does not have a pi bond]

One p orbital per atom max!