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JEE Main levelSingle correct optionDifficulty 2 of 3g1037

Which of the following is a necessary condition for resonance?

  1. AThe contributing structures must differ in the number of unpaired electrons.
  2. BSigma electrons must be shifted between the structures.
  3. CThe contributing structures must differ in the arrangement of the atoms.
  4. DAll the atoms involved in the delocalisation must lie in one plane, with parallel p orbitals overlapping.

Answer: D

Resonance needs overlapping parallel p orbitals, which requires a planar arrangement of the atoms involved. Contributors have the same positions of nuclei and the same number of paired electrons, and only pi electrons and lone pairs are moved.
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