Evidences show that nitrogen is attached to one of the oxygen atoms by a double bond and to the other by a dative bond. The resonance hybrid is shown as under which confirms the spectroscopic evidence that both nitrogen – oxygen bonds have same bond length.
Out of three sp2 hybrid orbitals of nitrogen one overlaps with alkyl group and two with oxygens while the unhybridized p orbital of N – atom containing a pair of electrons and lying perpendicular to the plane of hybrid orbitals overlaps sideway with half filled 2 p – orbitals of two oxygen atoms.
This forms π- bond above and below the plane of molecule.
Illustration 1. p-chloronitrobenzene has less dipole moment (2.4D) than p-nitrotoluene. Explain.
In p – chloronitrobenzene, the two vector dipole moment act opposite to each other which partially cancel each other. On the other hand, in p – nitro toluene, the two vectors acts in the same direction to give additive dipole resultant.
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