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Orbital interactions

  1. Oct 27, 2015 #1

    Titan97

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    Recently, I read two chapters from March's advanced organic chemistry. I came across gauche effect and SN2 reactions. In both phenomena, ##\sigma-\sigma^*## orbital interactions is involved. In gauche effect present in 1,2-diflouroethane, the C-F bonding orbital becomes an antibonding orbital so that C-H ##\sigma## orbital can overlap with the empty C-F ##\sigma## orbital.
    Capture2.PNG

    In SN2 mechanism, C-X bonding becomes antibonding orbital so that the nucleophile can attack the empty ##\sigma## orbital of Carbon
    Capture.PNG

    Am I correct? Is there any flaw in my understanding?
     
  2. jcsd
  3. Oct 27, 2015 #1

    DrDu

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    No, it is just that electron density from a near by orbital (e.g. the filled orbital on Nu) is brought into the anti-bonding orbital (C-X). This compensates the bonding by the filled C-X sigma orbital. It is not so that the latter one is emptied somehow.
     
  4. jcsd
  5. Oct 27, 2015 #2

    Titan97

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    C-X bond has a bonding and an antibonding orbital. Initially 2 electrons are filled in BO. That is, if the reactant was CH3Cl, then a +sp3 orbital of C and a p-orbital of Cl would overlap. This forms BMO. But how can C-Nu bond form when C-Cl ABMO is filled?

    Also, how can BMO and ABMO be formed simultaneously? Its given everywhere that 2 orbitals overlap to give two molecular orbitals. But each MO requires a different configuration of orbital.
     
    Last edited: Oct 27, 2015
  6. Oct 28, 2015 #3

    DrDu

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    O-chem books are not a good place to learn about MO-theory.
     
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