[xiankai]

while i know that aromatic hydrocarbon refers to benzene or a hydrocarbon with a benzene ring and my topic is abit going in circles, what i meant to ask is that, why isnt there C₅H₅, or something like that?

after all, it only involves delocalisation of the pi electrons as far as i know.

am i missing anything out like the delocalisation energy of such theoretical forms?

[tamim83]

To answer your question, yes there is a C5H6, but it is not aromatic.  There are more criteria for being aromatic than having delocalized pi electrons.  There also has to be  4n+2 pi electrons.  So C5H6, has 4 pi electrons (2 double bonds) and is therefore not aromatic.  Also, the molecule has to be planar.  All of these criteria are needed for a molecule to be aromatic.  Oh yeah, it should be cyclic too, almost forgot.   

Side note, if you remove a proton from C5H6 to make an anion, the resulting lone pair of electrons in (I am presuming) a p-orbital that can participate in delocalization.  So it gives the C5H5 anion 4n+2 pi electrons and makes the planar, cyclic anion aromatic, although the neutral molecule is not. 

[xiankai]

mm this is very interesting, in school we only learn what is 'relevant', hah. 

what is this magical number that confers aromaticy, 4n + 2?

by playing with numbers i figure that there has to be an odd number of electrons above and below, since (4n + 2)/2 gives u an odd number everytime, hehe.

[tamim83]

what is this magical number that confers aromaticy, 4n + 2?

I think it is called Huckel's rule, I have no clue how he came about it though.  It seems to "work" though. 

[victor]

One of my basic organic chemistry book said:
"why 4n+2? this formula can be explained with a modern theory of molecular orbital which we won't have it discussed here in this book.." 

[maxyoung]

Excellent explanation from tamim83. Just one point to remind you guys-the neutral species should be C5H6 not C5H5.  It's not possible to have odd number of hydrogen atoms in hydrocarbons unless it is charged

[stoneburner]

..or a free radical.

[Yggdrasil]

mm this is very interesting, in school we only learn what is 'relevant', hah. 

what is this magical number that confers aromaticy, 4n + 2?

by playing with numbers i figure that there has to be an odd number of electrons above and below, since (4n + 2)/2 gives u an odd number everytime, hehe.

As victor said, the answer lies in molecular orbital theory.  I'll try to offer a simple explanation based on example, showing why benzene's molecular orbitals (which has 6 = 4n + 2 pi electrons) are stable when conjugated but cyclobutadiene (which has 4 = 4n pi electrons) is not stable.

Benzene's conjugated pi system is a combination of six p-orbitals, one from each carbon.  This means that the combination of these six atomic orbitals must form six molecular orbitals.  From symmetry arguments, three of these molecular orbitals are bonding orbitals and three are antibonding.  Since the system contains six pi electrons (one from each carbon), all six electrons populate the three bonding orbitals.  Since all of the bonding orbitals are completely filled and there are no electrons in the antibonding orbitals, benzene is stable.

Cyclobutadiene's conjugated pi system is a combination of four p-orbitals, and therefore consists of four molecular orbitals.  By symmetry arguments, there is one bonding orbital, two degenerate non-bonding orbitals (i.e. the two non-bonding orbitals are of the same energy), and one antibonding orbital.  Since cyclobutadiene has four pi electrons, the first to fill the bonding orbital.  However, because the orbitals with the next highest energy (the non-bonding orbitals) are degenerate, one electron goes into each orbital (by Hund's rule).  Therefore if cyclobutadiene forms an aromatic system, it becomes a highly unstable diradical species.  Since having these two unpaired electrons is very energetically unfavorable, cyclobutadiene will not form an aromatic system and will instead deform so that the Pi orbitals are no longer conjugated.

For a visual of the molecular orbital energy diagrams of benzene and cyclobutadiene see:
http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Aromaticity/OrbitalArrays.GIF

[FeLiXe]

the interesting thing is that the energy scheme for monocyclic molecules always looks like the molecule standing on its corner (that's because of the Eigenvalues of the Hückelmatrix)

I drew that for German Wikipedia: http://de.wikipedia.org/wiki/Bild:ArESchema.jpg

[Mitch]

I wrote a nice article on the matter a while back ago. http://www.chemicalforums.com/index.php?op=Articles;article=19

[HP]

Just one fun comment on  von Kekule's dream:
http://members.optusnet.com.au/~charles57/Creative/Brain/kekule.htm
It's very hard (and painfull) for snakes to square their bodies but most common to circle
Again man is as grat as his dreams

[xiankai]

I wrote a nice article on the matter a while back ago. http://www.chemicalforums.com/index.php?op=Articles;article=19

that is interesting. are you going to make a follow-up article about more on it?  so that it'll have a 'happy' ending too