Hybridization of atoms in heterocyclic compounds

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Discussion Overview

The discussion centers on the hybridization of atoms in heterocyclic compounds, particularly in relation to aromaticity. Participants explore the hybridization states of various atoms, including nitrogen, oxygen, and sulfur, and how these relate to their electronic structure and bonding in aromatic systems.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant expresses a desire to understand how to determine the hybridization of heteroatoms in aromatic compounds, specifically questioning the sp2 hybridization of nitrogen and others.
  • Another participant suggests that sp3 hybridized nitrogen can be considered sp2 hybridized when conjugated to a pi-bond, indicating a preference for sp2 geometry in such cases, using pyrrole as an example.
  • Some participants argue that hybridization is not an inherent property of atoms but rather a conceptual tool to describe electronic wavefunctions, noting that different hybridizations can yield similar binding energies and atomic distances.
  • There is a discussion about whether hybridization defines coordination or vice versa, with some participants asserting that neither is definitive.
  • One participant emphasizes that it is often sufficient to specify the coordination of an atom without ascribing a hybridization state, suggesting that the choice of hybridization is largely conventional.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the necessity or utility of hybridization as a concept in describing atomic structure in heterocyclic compounds. Multiple competing views on the role and definition of hybridization remain present throughout the discussion.

Contextual Notes

Participants highlight that the discussion is influenced by differing theoretical frameworks, such as Valence Bond Theory and Molecular Orbital Theory, which may affect interpretations of hybridization and bonding.

Mayhem
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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?
 
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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.
 
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.
 
DrDu said:
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?
 
Neither
 
DrDu said:
Neither
Then what?
 
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.
 

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