Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Kryptid on 13/02/2004 21:31:16

Title: Confusion over coronene
Post by: Kryptid on 13/02/2004 21:31:16

In my chemistry lecture class, I recently learned that aromatic molecules must have an odd number of electron pairs in their pi-systems. If the molecule has an even number of electron pairs, then it is anti-aromatic if it is planar, or non-aromatic if it is not planar. This brings me to my question: Why is coronene aromatic?

Coronene is a hydrocarbon that is made of six benzene rings fused together in a shape similar to a snowflake. It's formula is C24H12. The molecule has a pi-system containing 12 electron pairs, which is NOT an odd number. Judging from this, coronene should be either anti-aromatic or non-aromatic. However, it is actually aromatic.

Can someone explain why coronene is aromatic?

Title: Re: Confusion over coronene
Post by: Ylide on 13/02/2004 22:55:18

By definition, coronene shouldn't be aromatic, but every source I can find says it is.  Weird.  If you find out why, please do post it.



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Title: Re: Confusion over coronene
Post by: Kryptid on 14/02/2004 00:42:02

I read that some polycyclic aromatic compounds can disobey the Huckel rule, such as pyrene. I have been unable to find any reason for this deviant behavior though.

Title: Re: Confusion over coronene
Post by: Ylide on 13/03/2004 08:43:54

Hey SC, this is kinda late but I happened to mention this to one of the organic chemistry profs.  He said that Huckel is really only "always" true for single ring systems.  For polycyclic ring systems, the true test for aromaticity comes from how the hydrogens near the conjugated pi system(s) behave in a proton NMR.  If they experience a downfield shift, it's an aromatic molecule.

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