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Aromaticity of ortho substituted benzene

  1. Jul 27, 2016 #1
    Are ortho substituted benzene rings aromatic in nature as the planarity in such compounds breaks due to steric hindrance ?
     
  2. jcsd
  3. Jul 28, 2016 #2

    Fervent Freyja

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    This is a poorly phrased question- it makes little sense to me.
     
  4. Jul 29, 2016 #3

    HAYAO

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    This is indeed a poorly phrased question. I am going to try my best interpreting the question and rephrase it:
    "Does steric hindrance by two substituent in ortho conformation cause change in aromaticity due to partial breaking of planarity?"

    Is this your question? If so, then yes. Aromaticity purely based on planarity is subject to change in presence of two substituents in ortho conformation. Most of the time, this leads to some degree of loss in aromaticity. This means that the energy as a molecule increases, π-π* transition is blue shifted, and the oscillator strength of this transition is decreased.

    However, I believe that the effect of the substituent itself does more than the effect based on planarity. For example, Substituents like fluorine, carboxylate, etc. extends the π-conjugation. In my intuition, even methyl substituent, known to have only the inductive effect, can increase the energy of π-conjugation and increase the oscillator strength due to breaking of symmetry.
     
  5. Aug 1, 2016 #4
    Even though planarity is distorted by steric hinderance, the overlap of the pi bonds remains. The bond angles have changed, and the molecular orbitals will be distorted, thus affecting resonance.Bond angles on SP2 are 1200, giving maximum overlap of the pi orbitals. Overlap is reduced when bond angles change.
     
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