Why is hybridization different in a ring? [duplicate]








**This question already has answers here**:
 
 




[Why is the lone pair of pyridine's nitrogen atom not part of the aromatic ring?](/questions/50266/why-is-the-lone-pair-of-pyridines-nitrogen-atom-not-part-of-the-aromatic-ring)

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[sp² hybridized orbital](/questions/27309/sp%c2%b2-hybridized-orbital)

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Closed last year.



Why is nitrogen $sp^3$ hybridized in a compound such as NH3 but $sp^2$ hybridized in something like pyrrole, which has a ring structure? In both cases there are three bonds and one lone pair.




One might argue that the nitrogen in pyrrole does not really have a lone pair we can identify like that in ammonia or for that matter pyrrolidene ($\ce{C5H9N}$, adding enough hydrogen atoms to saturated the five-membered ring in pyrrole). In the pyrrole molecule, what "should be" the lone pair on nitrogen is instead coupled with the other pi electrons to form a continuous ring of conjugated pi bonding. Such a coupling is commonplace when it forms an aromatic ring, meaning in the most common cases $4n+2$ electrons in this ring (like the six pi electrons in pyrrole). There are also cases where what seems to be an *empty* orbital really isn't empty because of a similar coupling (like the [cyclopropenyl cation](https://en.wikipedia.org/wiki/Cyclopropenium_ion)). You should see more examples of this effect as you study aromatic compounds.


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