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Author Topic: Aromaticity of Pyridine and Pyrrole  (Read 21226 times)

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a confused chiral girl

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Aromaticity of Pyridine and Pyrrole
« on: December 13, 2006, 11:38:35 AM »

I am confused about p-orbitals. I know that all double bonds have 1 p orbitals, but are all lone paris also in the p orbital or in s orbital?  I am reviewing my notes, and apparently both pyridine and pyrrole are aromatic compounds. Pyridine fits all: cyclic, conjugated at every atom in the ring, planar, and also Huckel's Rule. For pyrrole, isn't the N that is attached to the hydrogen sp3?? For some reason, I have made special notes saying that it is sp2 and not sp3, which doesn't make sense at all.

ps. are those lone pairs that I drew around the pyridine ring correct, or is there suppose to be Hydrogens bonded there even not it is not explicitly written in the structure?
thank you.
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Yggdrasil

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Re: Aromaticity of Pyridine and Pyrrole
« Reply #1 on: December 13, 2006, 01:23:56 PM »

The lone pairs in pyridine are not correct, they should be hydrogens.  Note that hydrogens generally aren't explicitly written in most organic structures.

Anyway, lone pairs usually occupy a hybrid orbital.  For example, in NH3, the N is sp3 hybridized and the lone pair occupies one of the sp3 orbitals.  So, one would expect the nitrogen in pyrrole to be sp3 hybridized.  However, in pyrrole, the nitrogen actually takes on an sp2 hybridization in order to allow its lone pair to become delocalized in the aromatic system.  Since aromaticity stabilizes the molecule by so much, there is a huge driving force to put the lone pair in a p orbital instead of an sp3 orbital.
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a confused chiral girl

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Re: Aromaticity of Pyridine and Pyrrole
« Reply #2 on: December 13, 2006, 02:48:42 PM »

  However, in pyrrole, the nitrogen actually takes on an sp2 hybridization in order to allow its lone pair to become delocalized in the aromatic system.  Since aromaticity stabilizes the molecule by so much, there is a huge driving force to put the lone pair in a p orbital instead of an sp3 orbital.

thanks. but then, how come the lone pair on the N in the pyridine structure is part of the p orbital conjugation to test for aromaticity? How were you able to tell right away that the lone pair in the Pyrrole is not part of the rules for determining whether the structure is aromatic or not?
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Yggdrasil

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Re: Aromaticity of Pyridine and Pyrrole
« Reply #3 on: December 13, 2006, 05:58:47 PM »

In pyridine, the nitrogen is sp2 hybridized because it has a double bond.  The one electron in its p-orbital contributes 1 electron to the 4n+2 electrons in the aromatic ring.  In the case of pyridine, the lone pair occupies a sp2 orbital which lies perpendicular to the conjugated pi system.  Therefore, the lone pair in the nitrogen of pyridine does not contribute to the aromatic system.

In pyrrole, one would expect the nitrogen to be sp3 hybridized because it has three single bonds.  If this were the case, the lone pair would lie in a sp3 orbital.  However, it is favorable for the nitrogen to change its hybridization to sp2 and place the lone pair in a p-orbital.  In the p-orbital, the lone pairs can become delocalized and form a conjugated pi system with the other pi-bonds in pyrrole.
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