terbium (Old P2P Cook)
09-03-02 01:16
No 351993
      Coronene and Huckel's rule.  Bookmark   

Coronene is said to be aromatic and indeed it is cyclic, planar and every carbon atom is paired with a conjugated double bond. But coronene has 24 carbon atoms/12 double bonds yielding 24 pi bonded electrons. This does not fit Huckel's rule of 4n+2 pi bond electrons. Am I correct that coronene violates Huckel's rule?
 
 
 
 
    Chromic
(Synaptic Self-Mutilator)
09-03-02 01:41
No 352002
      Huckel's rule holds  Bookmark   

Damn, I was going to try my Huckel MO calculator on this hexacyclic symmetrical structure... but alas, the program has a maximum of 20 atoms. <sigh>

Coronene is aromatic, just like porphine and meso-tetraphenylporphyrin. Huckel's rule is used on the atoms in the aromatic system (ie even tho in meso-tetraphenylporphyrin there's a ton of double bonds and lone pairs in the phenyl and nitrogen groups, there's just 9 pairs of electrons in the aromatic ring).

Keep in mind every proton inside the electromagnetic ring is highly antiaromatic, and if you ever see it on NMR you'll see the protons highly shielded and way upfield of tetramethylsilane (eg ~-3 ppm).
 
 
 
 
    terbium
(Old P2P Cook)
09-03-02 02:37
No 352019
      Pyrene too.  Bookmark   

I don't follow your statement that Huckel's rule holds. I am only counting conjugated double bonds. Are you saying that part of the coronene molecule is not aromatic? I think that every carbon in coronene is aromatic.

Pyrene with 16 carbon atoms also violates Huckel's rule. It would seem that Huckels rule only applies to simple cyclic systems.
 
 
 
 
    foxy2
(Distinctive Doe)
09-03-02 03:31
No 352046
      Exception to Huckel's Rule  Bookmark   

Huckelís rule has been expanded to cover fused polycyclic compounds because when these compounds have the requisite number of electrons, they also show unusual stability.

In fused ring systems, only the electrons located on the periphery of the structure are counted when Huckelís rule is applied.

See Page 28 here
http://www.aptexts.com/books/millerwrm/MillerCH1.pdf

Those who give up essential liberties for temporary safety deserve neither liberty nor safety
 
 
 
 
    Chromic
(Synaptic Self-Mutilator)
09-03-02 03:35
No 352049
      Pyrene works too  Bookmark   

Pyrene works too. I count 7 electron pairs in the outer ring, n=3, ie 3*4+2 = 7*2. It's aromatic. The thing is, you must read Huckel's rules carefully. I don't have them at my finger tips, but when using the rule, you must have a cyclic arrangement of conjugated double bonds or lone pairs in the ring system (this doesn't have to be with carbon, any heterocycle works, sulfur, nitrogen, oxygen, etc)

Draw an moledular orbital model if in doubt (or use any freeware software to calculate what you seek)...
 
 
 
 
    slappy
(Hive Bee)
09-03-02 05:43
No 352094
      Antiaromatic  Bookmark   

Coronene is antiaromatic. 4n+2 electrons is aromatic, and 4n is antiaromatic. Compare with compounds like Cyclobutdiene or Cylcooctatetraene. Usually, you would say that a heavily unsaturaed ring would be aromatic, antiaromatic, or non-aromatic.

An antiaromatic ring would be one where you have a fully conjugated pi system, but 4n electrons instead of the magic 4n+2. Antiaromatic compounds don't benefit from the same stabilization energy that aromatic compounds do.
 
 
 
 
    terbium
(Old P2P Cook)
09-03-02 07:04
No 352121
      I can get this result now.  Bookmark   

Pyrene works too. I count 7 electron pairs in the outer ring, n=3, ie 3*4+2 = 7*2. It's aromatic.
I can get this result now, 7 exterior double bonds for pyrene and 9 for coronene, but it is dependent on how you draw the location of the double bonds. With a different arrangement of alternating single and double bonds I get 6 exterior double bounds for both pyrene and coronene. In the arrangements that give the value of 6 exterior double bonds, the interior double bonds are not fully interior but connect between one fully interior carbon and one exterior bridgehead carbon. It would seem that the rule for drawing the correct structure is to first try to place double bonds between pairs where both carbons are interior and then work to the outside.

So, does this mean that pyrene has one double bond that is not aromatic and has different chemical reactivity while coronene has 3 non aromatic double bonds? Actually the 3 interior double bonds in coronene obey Huckel's rule seperately from the 6 exterior double bonds, does this mean that coronene has two aromatic systems?
 
 
 
 
    foxy2
(Distinctive Doe)
09-03-02 08:06
No 352147
      What?  Bookmark   

does this mean that coronene has two aromatic systems?

The Huckel Rule is just a numbers game that works for 99% of all delocalized orbital systems.  It doesn't really have significance in a literal sense.

does this mean that pyrene has one double bond that is not aromatic and has different chemical reactivity

No because neither pyrene nor coronene has any double bonds at all.  The whole system is delocalized, and there is no way you can describe it with single or double bonds. 

Those who give up essential liberties for temporary safety deserve neither liberty nor safety
 
 
 
 
    Chromic
(Synaptic Self-Mutilator)
09-13-02 04:51
No 355788
      Yep  Bookmark   

Yep, they're all delocalized thru the conjugated ring system. Just like protoporphyrin IX (when chelated with iron, heme b). Although it looks like separate aromatic systems on paper, the electrons are spread thru the structure.

Huckel's rule can be adapted for these more complicated conjuated ring systems tho, right? (couldn't one use a Huckel matrix and figure this out?) Or am I just plain wrong? Is there another way to determine if there's aromatic stability?