aromaticity

[vixen]

can someone help me understand this?

from what I understand, for something to be aromatic, it has to have 3 things, 1) must follow the 4n+2 rule 2) be planar 3) have a closed loop (meaning all sp2 carbons)

How can you tell if something is going to be planar? Isn't that synonymous w/having sp2 carbons, or do anions and cations make a difference.

Also, if something is ANTIaromatic, it follows the same thing as aromatic except its 4n?

and nonaromatic is everything that doesn't follow these rules?

I really am lost as to how to draw molecular orbitals and then be able to tell if its aromatic or not...also, sometimes the answers will say, "its non aromatic because the internal H's prevent planarity"...how do you know if the H's are internal?

Ok, if anyone wants to help me out, I'd really appreciate it...mechanisms I understand, but this stuff I really don't

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[ItsGavinC]

simseema,

Yes, to be planar, the molecule must have sp2 carbons, so to restate your rules:

1. The molecule must have an uninterrupted cyclic cloud of pi electrons (must be cyclic/every atom must have a p orbital).

2. The pi cloud must contain an odd number of pairs of electrons (or Huckel's rule).

A nonaromatic compound has an exception to one of the above points (ie, perhaps it is cyclic, has an odd # of pairs of electrons, but does not have an uninterrupted cyclic cloud of pi electrons).

An antiaromatic compound is one that is less stable than an analogous cyclic compound with localized localized electrons.

In regards to aromatic vs. antiaromatic stablilities, they are as follows, with the increasing stability being on the left: aromatic compound > cyclic compound with localized electrons > antiaromatic compound

Hope this helped somewhat.

 

[vixen]

thanks gavinc, now I understand what you mean by the "uninterupted pi system"...that was helpful

ok, I think aromaticity is coming to me, now its just those (*&^%& molecular orbitals! does anyone know how to draw one, lets say for toluene (aka, methyl benzene-I'm making up an example), and then just by looking at the MO, figuring out if its aromatic/anti or non?

Thanks again, I appeciate it.

 

[Buckyball]

Simseema,

Remember, an aromatic compound must have a CONJUGATED system (i.e. alternating double and single bonds).

Also, it must follow Huckel's rule. Although aromatics are planar, it would be too hard to tell from the structure if it is planar. If you stick to these rules, you should be fine.

Bucky

 

[vixen]

thanks you guys...I understand already about the conjugated system, etc, but moreso was wondering about when you have O's or N's in the ring, and sometimes they're either anions or cations. Then you have to consider if the pi electrons are are part of the pi system or if they're not counted. If it's an anion, I think one pair of electrons are included in the pi bonds, but if its a cation then I don't know what happens...

the stupid MO's...from looking at it, does anyone know how to tell aromaticity from a MO diagram? Thanks!

 

[DMD DOC]

Never quite understood why Organic Chemistry was so important for pre-dents, I think the time would be better spent learning biochemistry.

- Vikas

 

[vixen]

I think thats true, and as strange is it may sound, I kinda like organic Biochem on the other hand, wasn't much fun, but I had the devil herself teaching it <img border="0" title="" alt="[Eek!]" src="eek.gif" />

 

[Buckyball]

Hey Simseema,

From what I know, if you have a cyclic anion, the extra 2 electrons is included in the pi-system. For example, look up the structure of cyclopentadiene. There are 4 electrons in the cycle. However, if you deprotonate with base, you remove one hydrogen, and the electrons go into the cyclic system, giving the stable cyclopentadienyl anion, with 6 pi electrons (4(1)+2 = 6), which makes it aromatic.

In the case of heterocyclic compounds, if you look at pyridine for example, it's like benzene with one CH replaced by Nitrogen. It still has 6 pi electrons. If you look at pyrrole, which is like cyclopentadiene with one CH removed, and NH put in its place. there seem to be only 4 electrons, however it is aromatic, because a lone pair on Nitrogen (it's not a cation or anion) contributes 2 electrons, making it aromatic.

Hope this helps!

Buckyball

 

[vixen]

got it, thanks buckyball...

btw, did you get your name from buckminister fullerine? that soccerball structure?

 

[Dentista]

my sympathy goes out to you guys studying for this stuff..

 

[Buckyball]

I sure did get my name from C60... that crazy compound!

Bucky