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Delocalised pi electrons in benzene

  1. Sep 14, 2010 #1
    Why do pi electrons in a benzene move?
     
  2. jcsd
  3. Sep 14, 2010 #2

    alxm

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    In what sense? They don't really move more or less than other electrons. (unless there's a magnetic field, in which case you get a ring current, which is cool)
     
  4. Sep 14, 2010 #3
  5. Sep 15, 2010 #4

    DrDu

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    Granpa, I tried to answer the question in the other thread.
     
  6. Sep 15, 2010 #5

    alxm

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    Resonance doesn't actually mean that the structure is moving back and forth between the different structures; they're not true reaction arrows. Rather, (in the VB theory picture) they're in a quantum superposition of the various resonance structures - all at the same time, but to different extents.

    As for NO2, 5 + 6 +6 = 17 valence electrons. It can't possibly obey the octet rule with an odd number of electrons, since one of them must be unpaired. It's a radical.
     
  7. Sep 15, 2010 #6

    Ygggdrasil

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    Are you familiar with the particle in a box in quantum mechanics? One very important principle that this model system illustrates is that as the size of the box increases, the ground state energy of the electron decreases. Therefore, when an electron has an opportunity to "spread out" over a larger area it does so in order to lower its total energy.
     
  8. Sep 19, 2010 #7
    Think of it this way: different resonance structures aren't descriptions of where electrons are, just where the electrons could be according to their wave functions.
     
  9. Sep 19, 2010 #8
    I read this question as "Why does benzene have a ring of delocalised electrons rather than double bonds", the answer to which is:

    Double bonds form when two pi orbitals which are next to each other overlap. However, in benzene, each pi orbital has two other pi orbitals next to it and hence, doesn't really form a bond with either of them, but instead forms a bond with both. This happens to all 6 pi orbitals in the molecule and you end up with a hexagonal/circular ring of delocalized electrons. Seeing as this ring is essentially just 1 big orbital, the electrons in each carbon's pi orbital can technically be anywhere in the ring, i.e they move around the ring.

    Hope that helped.
     
  10. Oct 26, 2011 #9
    I don t understand.Pi electrons are more movable,aren t they?This is princip of the resonance,isn it?Pi electrons can easily move.Correct me if I am wrong.
     
  11. Oct 26, 2011 #10

    Ygggdrasil

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    As alxm said, they don't move any more than any other electrons. Electrons involve in a chemical bond move around in some confined area defined by the molecular orbital they inhabit. Most of the time, the molecular orbital is confined between two atoms. In the case of conjugated pi systems like benzene, the molecular orbital spans multiple atoms. So, the electrons are free to around in a larger volume, but they do not move any faster than electrons involved in other types of chemical bonds.

    As for why the electrons are able to occupy a larger volume, see my explanation above. Molecular orbitals with larger volumes have lower energies than molecular orbitals with smaller volumes. Therefore, if given the chance, neighboring orbitals will combine to form larger orbitals.
     
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