Aromatic?

[yangxx2]

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How is this molecule aromatic?

http://img851.imageshack.us/i/img2011060400057.jpg/

Thanks!

 

[loveoforganic]

The general rule is that if an sp3 hybridized atom with a lone pair would give a molecule aromaticity by rehybridizing to sp2 and putting that lone pair in a p orbital, it will. It confers a whole lot of extra stability

 

[coyotelime]

My orgo prof kind of touched on these weird cases. I don't think the compound is aromatic, but if it were, the only plausible explanation is that since the electrons move around within the p orbitals, somehow the p-orbitals of sulfur can interact with the remaining p-orbitals of the cyclic compound? But that wouldn't make sense because p-orbitals interact by side on overlapping.

I think the answer may be wrong.

Edit: Some nitrogen cases happen when the lone pair on the nitrogen is actually located in the p(x) orbitals of the atom, in which case it doesn't interact with the remaining p-orbitals of the cyclic compound. But either way, the atom has to be sp2 hybridized so there is a free p orbital to interact, and as biophilia said, the sulfur atom is clearly sp3 hybridized.

 

[loveoforganic]

What makes you say it's clearly sp3 hybridized? This is the sulfur analogue of furan, an aromatic heterocycle with oxygen in place of sulfur. The oxygen in furan is sp2, and the molecule is aromatic.

In the diagram on the right, the second lone pair is in an (undrawn) sp2 orbital, in the plane of the ring (perpendicular to the p-orbitals). As you go further down the column, iirc, you do get poorer overlap due to the size of the heteroatom, but I'm not sure where the aromaticity cutoff is. At any rate, thiophene is definitely aromatic

http://en.wikipedia.org/wiki/Thiophene

 

[yangxx2]

One of the lone pairs on the sulfur is conjugated with the pi bonds of the ring, so this compound has a total of 6 pi electrons in the ring (4 from the 2 pi bonds and 2 electrons from one of the lone pairs on sulfur), which follows Huckel's rule.

If you replace the sulfur with a N-H, you will have a similar situation: the lone pair on the nitrogen is involved in the aromatic ring (and consequently unavailable to abstract a proton in an acid-base reaction).

The one weird thing about this compound is that normally I would say the sulfur must be sp3 hybridized, but here, if the sulfur is sp3 hybridized (as opposed to sp2 hybridized) then the compound could not be aromatic. If this compound is aromatic, then I think the sulfur must be sp2 hybridized even though it is attached to 4 "things" and normally would be thought of as sp3 hybridized. I think there are some cases where this occurs also with nitrogen (i.e., nitrogen "should" be expected to be sp3 hybridized but in reality is sp2 hybridized). I'm not really sure about this last part, but as for why the compound in your picture is aromatic -- it's because one electron pair on sulfur is used in the ring.

The general rule is that if an sp3 hybridized atom with a lone pair would give a molecule aromaticity by rehybridizing to sp2 and putting that lone pair in a p orbital, it will. It confers a whole lot of extra stability

Great, thank you so much! Got it! 👍