Organic Chemistry - why not just three 2p orbitals in ethene?

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Discussion Overview

The discussion revolves around the hybridization of orbitals in ethene, specifically questioning the necessity of sp2 hybridization versus the use of three unhybridized 2p orbitals for bonding. The scope includes theoretical considerations in organic chemistry and the implications of bonding geometry and energy levels.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that after promoting an electron, the hybridization of the 2s and two 2p orbitals into three sp2 orbitals is necessary for effective bonding.
  • Others argue that while three orthogonal p orbitals could theoretically form bonds, they are at a higher energy level compared to the sp2 orbitals, raising questions about the energy considerations in bonding.
  • A participant suggests that the sp2 hybrid orbitals have lower energy levels, which makes them preferable for bonding over the unhybridized p orbitals.
  • Another viewpoint emphasizes the transition from valence bond theory to hybridization theory, asserting that hybridization allows for more effective bonding due to the geometry of orbital overlap.
  • One participant challenges the symmetry argument against sigma bonding between unhybridized p orbitals and unhybridized s orbitals, suggesting that symmetry may not be a valid concern in this context.

Areas of Agreement / Disagreement

Participants express differing opinions on the necessity and implications of hybridization in ethene. There is no consensus on whether the symmetry argument against using unhybridized p orbitals is valid, and the discussion remains unresolved regarding the energy considerations and bonding effectiveness.

Contextual Notes

Limitations include the dependence on definitions of hybridization and bonding types, as well as unresolved questions about the energy levels of orbitals and their implications for bonding geometry.

lifeiseasy
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After promoting one of the electrons in the 2s orbital, the 2s orbital and two of the 2p orbitals undergo hybridization to form three sp2 orbitals, leaving the other 2p orbital unhybridized. But why is the hybridization necessary? Isn't the three 2p orbitals sufficient to form three identical sigma bonds with carbon and hydrogen?
 
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Yes, three othogonal p orbitals woud be sufficient to form 3 bonds but each of those p orbitals lie at slightly higher energy than the s orbital. Why would it be necessary to bond to those three higher energy orbitals due only to an argument of geometry. Isn't energy more important?
 
What you mean is the sp2 hybrid orbitals have lower energy levels than the p orbitals... So the molecule opts for the hybrids... Thanks!
 
I think this is the revolution from valence bond theory to a better, precise hybridization theory. From energy concerns, this kind of overlapping/hybridization can have better and effective bonding than just simply let the orbitals "bind" together.

and, by definition, sigma bond and pi bond should be defined under hybridization.
sigma bond means head-on overlapping and pi bond means side-way overlapping.

Clearly , 3 2p orbitals cannot do that, owning to symmetry.
 
warenzeichen said:
Clearly , 3 2p orbitals cannot do that, owning to symmetry.

I don't believe that a symmetry argument could be made against sigma bonding between unhybridized p orbitals and unhybridized s orbitals.
 

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