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Resonance and Aromaticity

  1. Feb 12, 2012 #1
    I have two doubts in my course regarding resonance and aromaticity....

    (1) In various resonating structures, the hybridization of an atom may vary. Eg:- in aniline's nitrogen atom, it is sp3 as well as sp2 in different resonating structures. Since the actual molecule is a weighted mean of all the resonating structures, the hybridization of the nitrogen atom must be somewhere between sp2 and sp3 that is, the hybrid orbitals have sp3 as well as sp2 character. Now, I want to know if the molecule is planar or not. If the hybridization of the nitrogen atom was purely sp3, the molecule would be non-planar and if it was purely sp2. So how do I decide it's planarity?

    (2) When we check for aromaticity, the molecule must be planar. However, is this 'planar' condition only for the atoms involved in the delocalization of electrons, or for all atoms?
     
  2. jcsd
  3. Feb 13, 2012 #2

    chemisttree

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    The only resonance structures you can draw that have the aniline's nitrogen lone pair involved in delocalization are structures where you generate a separation of charge... positive on the nitrogen and negative at the ortho and para positions. You can argue that the nitrogen might direct an electrophile to those locations since the transition state would be favorable but the transition state wouldn't be aromatic. Even so, why are you obsessed regarding planarity? Do you think the nitrogen of aniline does not lie in the plane of the aromatic ring? It does but that doesn't prove that it's lone pair is involved in the aromatic system (it's not, BTW). There are enough pi electrons (6) in the aromatic system without the need for aniline's pair.

    Only the delocalized system of electrons directly involved in the aromatic system need be planar. t-Butyl benzene is aromatic but is not planar everywhere.
     
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