Hybridization of atoms in heterocyclic compounds

[Mayhem]

TL;DR

How to determine whether a non-carbon in a heterocyclic compound is sp, sp2, or sp3 hybridized.

I'm trying to fortify my understanding of aromaticity in heterocyclic compounds. I understand that every atom in an aromatic compound must be sp2 hybridized (I don't like the "conjugated" definition), which is easy to spot for carbon, but how do I determine it for atoms such as nitrogen, oxygen, sulfur and so on?

 

[Mayhem]

I looked into it some more, and it seems that my confusion arose from the fact that seemingly sp3 hybridized nitrogen atoms can be sp2 hybridized if they are conjugated to a pi-bond. Apparently it's energetically more favorable to assume sp2 geometry in this situation. This explains why something like pyrrole is indeed aromatic.

 

[DrDu]

The atoms "are" not hybridized. It is you who decides to use one or the other hybridization to better describe the electronic wavefunction. For example, you can assume carbon to be sp3 hybridized or sp2 hybridized in ethylene and obtain almost the same binding energy and atomic distances. Also note that hybridization is a concept from Valence bond theory, while pi bonding is from molecular orbital theory. So the question is what you really learn from ascribing a hybridization tag to an atom? Usually it would be sufficient to specify the coordination of the atom (planar trigonal vs trigonal pyramidal) and not to talk about hybridization at all.

 

[Mayhem]

The atoms "are" not hybridized. It is you who decides to use one or the other hybridization to better describe the electronic wavefunction. For example, you can assume carbon to be sp3 hybridized or sp2 hybridized in ethylene and obtain almost the same binding energy and atomic distances. Also note that hybridization is a concept from Valence bond theory, while pi bonding is from molecular orbital theory. So the question is what you really learn from ascribing a hybridization tag to an atom? Usually it would be sufficient to specify the coordination of the atom (planar trigonal vs trigonal pyramidal) and not to talk about hybridization at all.

Yes, but in what direction is it defined? Does hybridization define coordination or the other way around?

 

[DrDu]

Neither

 

[Mayhem]

Neither

Then what?

 

[DrDu]

As I said, you can describe an atom in a given compound with different hybridizations. In the case of nitrogen, it is not even necessary to assume it hybridized at all. To describe planar coordinations with sp2 and $\psi$ tetraheldral ones with sp3 is mostly convention and not physical reality. As a chemist, you will have to know the coordination of the heteroatom and may from this deduce whether, it may participate in a delocalized bonding or aromatic structure. If you describe the corresponding structure with un-hybridized, sp2 or sp3 hybridized atoms won't make any difference to the properties you observe.