[mohit.shurtugal]

Huckel's rule for aromaticity is generally defined for a single ring system. However Solomons and Fryhle speaks about the aromaticity of Pyrene although it has 16 pi electrons. Solomons then speaks about disregarding the internal bond pi bond of pyrene and then considering it to be aromatic. however in napthalene or anthracene we dont neglect any internal pi bond then why do we do so in the case of pyrene.

[mohit.shurtugal]

Help Me out!!

[MissPhosgene]

In chemistry, there are a lot of after the fact rationalizations.

You have to remember, you are reading about what someone thinks and they can be wrong.

I think Solomon is trying to explain the reactivity of pyrene and it's stability relative to other molecules.

Draw resonance structures and maybe you can understand why the statement is being made.

Huckel's rule is just a model we use to explain, it isn't real.

[MrTeo]

Huckel's rule is just a model we use to explain, it isn't real.

In fact Huckel's Rule doesn't apply to pyrene like it doesn't work with many other policyclic molecules... I once read there's a way to prove its validity only on one or two ring using quantum mechanics...

If you draw the resonance structures as MissPhosgene suggested you'll easily see that if you put a double bond in the internal bond the aromaticity of the molecule is lost 

[408]

There is also a paper out there using the huckel rule for non-cyclic systems.  Groups such as guanidinium, can be said to be aromatic, but not cyclic!

It was a bit handwavey though...

[MissPhosgene]

I did not know people said that about guanidinium

I will find the paper... I wonder why they are making that claim? They could discuss anisotropy instead... maybe it's diatropic(excluding the ring part of ring current)

Maybe they could just say (I don't actually know what they said), that it has complete delocalization, akin to benzene.

That blows my mind. All the definitions I learned in first semester ochem and NMR class.. out the window.

[408]

I will add the citation here today or tomorrow, it is on my work computer.

[MissPhosgene]

Thanks 408!

[bromidewind]

Huckel's rule for aromaticity is generally defined for a single ring system. However Solomons and Fryhle speaks about the aromaticity of Pyrene although it has 16 pi electrons. Solomons then speaks about disregarding the internal bond pi bond of pyrene and then considering it to be aromatic. however in napthalene or anthracene we dont neglect any internal pi bond then why do we do so in the case of pyrene.

You ignore the internal bond because it doesn't participate in the aromaticity of the ring. Take it out, and you have [14]annulene which is also aromatic. Basic rule of thumb is that if the bond is internal, i.e. none of its carbons are on the outside of the ring, their pi-electrons are disregarded. Naphthalene and anthracene would lose their aromaticity if the bonds between opposite carbons were removed because the internal hydrogens would cause it to lose it's planarity. If I'm not mistaken, any polycyclic fused benzene compound is considered aromatic, but they lose stability as they get bigger and bigger. Clar's theory discusses why this is so, if you're interested. If you can get your hands on a copy of it, the Journal of Physical Organic Chemistry published an article entitled "Assessment of Clar's aromatic π-sextet rule by means of PDI, NICS and HOMA indicators of local aromaticity" in Vol. 18, Issue 8 of August 2005.

[408]

Gund, P. J. Chem. Ed 1972, 49, 2, 100-103