Aromatic, Anti-Aromatic, Planar....

[MTD52]

I have so much trouble determining what is aromatic and anti-aromatic. What I know is that to be anti-aromatic is has to have 4n pi electrons, and aromatic is 4n+2 pi electrons. That correct?

I don't really know how to figure out whether or not it is planar or how to determine if the electrons are "pi" or whatever else they might be. Double bonds are always pi, right? But I don't what to do with the unpaired electrons.

Help me out?

---

Members don't see this ad.

 

[AndyK]

I have so much trouble determining what is aromatic and anti-aromatic. What I know is that to be anti-aromatic is has to have 4n pi electrons, and aromatic is 4n+2 pi electrons. That correct?

I don't really know how to figure out whether or not it is planar or how to determine if the electrons are "pi" or whatever else they might be. Double bonds are always pi, right? But I don't what to do with the unpaired electrons.

Help me out?

All carbons need to be sp2 hybridized to be planar. If there is an sp3 carbon anywhere, it is non-planar and thus non-aromatic. If there is an N or O that's part of the ring, you'll count one of the lone pair as pi electrons (because they can resonate inside the ring) - for instance furan

 

[MTD52]

What about pyridine?

That has 3 double bonds plus a lone pair of electrons on the Nitrogen. According to what you just said, that would give it 8 pi electrons, but there are only 6, and it's aromatic.

Also, (I know this isn't the right naming, but that's irrelevant), but what about a conjugated cyclooctene. I did a problem and put that as an anti-aromatic compound and it was wrong, but I don't see why because it has 8 pi electrons?

 

[UCB05]

What about pyridine?

That has 3 double bonds plus a lone pair of electrons on the Nitrogen. According to what you just said, that would give it 8 pi electrons, but there are only 6, and it's aromatic.

Also, (I know this isn't the right naming, but that's irrelevant), but what about a conjugated cyclooctene. I did a problem and put that as an anti-aromatic compound and it was wrong, but I don't see why because it has 8 pi electrons?

In the case of pyridine, the "bonded" electron pair participates in aromaticity. the lone pair does not, and sits in an orbital perpendicular to the plane of the pi cloud.

cyclooctene is not planar, so it's non-aromatic, not antiaromatic. that's probably something you'll just have to remember =/

 

[MTD52]

In the case of pyridine, the "bonded" electron pair participates in aromaticity. the lone pair does not, and sits in an orbital perpendicular to the plane of the pi cloud.

cyclooctene is not planar, so it's non-aromatic, not antiaromatic. that's probably something you'll just have to remember =/

So there's no way to tell if something like that is planar, other than remembering? I mean I won't forget that exact one, but if a different molecule comes up...

I know that the bonded pair participates in that aromaticity, but I don't know why. Again, I can remember that for pyridine, but if I see a different molecule, I still have to know how to figure it out.

 

[loveoforganic]

You can't predict whether a large molecule will be planar or not.

 

[AndyK]

So there's no way to tell if something like that is planar, other than remembering? I mean I won't forget that exact one, but if a different molecule comes up...

I know that the bonded pair participates in that aromaticity, but I don't know why. Again, I can remember that for pyridine, but if I see a different molecule, I still have to know how to figure it out.

The cyclooctene is the only exception you have to know. Just one more tiny fact to memorize - just remember it looks like a boat.

 

[MTD52]

I won't forget now . Although I was looking in the Kaplan BB yesterday and it gives the cyclooctene as an example of an anti-aromatic compound. NOT GOOD lol.

My other question is still sort of unanswered too. In the case of pyridine (and other cyclic compounds with atoms other than C), how do you know what electrons participate in aromaticity? Like the double bonded N in pyridine, where only the double bond does and the free electrons don't. Or in a conjugated 5 member ring with S (forget the name), where the one pair of free electrons makes it aromatic. I mean I know a few just because I remember, but there are always answers that I have never seen before in the practice material.

 

[loveoforganic]

If the heteroatom is part of a double bond and it's lone pair is not part of that double bond, then the lone pair does not take part in the aromaticity of the molecule.

 

[b3thani]

Yeah, what "loveoforganic" said.

Okay like they said above. Aromaticity depnds on 4 things.

1. must be a ring
2. must be planar
3. can't be sp3 hybridized
4. follow huckel's rule

-If all three are true but number four, you have an ANTI-AROMATIC compound
-If any or all of the first three fail, then you have a NON-AROMATIC compound
-the only time you can have an sp3 hybridized atom, is if you have an atom with a lone pair that's adjacent to a double bond, like furan. So, if you have an atom with a lone pair, the way you can tell if the lone pair contributes to the pi system is whether or not the atom (with the lone pairs) is double bonded. If it is, then the lone pair DOES NOT contribute, because the p orbital is already being used to make the double bond. If it isn't, then only ONE pair of electrons will contribute, like in furan.

 

[MTD52]

Yeah, what "loveoforganic" said.

Okay like they said above. Aromaticity depnds on 4 things.

1. must be a ring
2. must be planar
3. can't be sp3 hybridized
4. follow huckel's rule

-If all three are true but number four, you have an ANTI-AROMATIC compound
-If any or all of the first three fail, then you have a NON-AROMATIC compound
-the only time you can have an sp3 hybridized atom, is if you have an atom with a lone pair that's adjacent to a double bond, like furan. So, if you have an atom with a lone pair, the way you can tell if the lone pair contributes to the pi system is whether or not the atom (with the lone pairs) is double bonded. If it is, then the lone pair DOES NOT contribute, because the p orbital is already being used to make the double bond. If it isn't, then only ONE pair of electrons will contribute, like in furan.

Right, I know the rules. It's just detrmining which electrons are pi that I'm having the trouble with.

As for the part that I bolded, doesn't that mean it participates in aromaticity either way? If it's double bonded, than the double bond has pi electrons. If it's not double bonded, one lone pair is pi electrons. So doesn't that mean that either way, there are 2 pi electrons on that atom?

 

[loveoforganic]

No. If it's involved in a double bond then that atom would only be contributing one electron to that pi bond.

 

[MTD52]

No. If it's involved in a double bond then that atom would only be contributing one electron to that pi bond.

Thanks a lot