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

  1. Aug 10, 2009 #1
    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?
     
    Last edited: Aug 10, 2009
  2. jcsd
  3. Aug 10, 2009 #2

    chemisttree

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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?
     
  4. Aug 10, 2009 #3
    What you mean is the sp2 hybrid orbitals have lower energy levels than the p orbitals... So the molecule opts for the hybrids... Thanks!
     
  5. Sep 7, 2009 #4
    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.
     
  6. Sep 9, 2009 #5

    chemisttree

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    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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