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JEE Main levelSingle correct optionDifficulty 2 of 3g1194

Which statement about the allyl cation, allyl radical and allyl anion, with charge +, one electron, or -, is correct?

  1. AThe allyl anion is aromatic because its pi system has 4 electrons
  2. BThe two C-C bonds of the allyl cation are of different lengths
  3. CThe three-carbon pi system contains 2, 3 and 4 electrons respectively
  4. DThe allyl radical has 4 electrons in the pi system

Answer: C

The three p orbitals hold 2 electrons in the cation, 3 in the radical and 4 in the anion. The two C-C bonds are equivalent in each species by resonance; 4 pi electrons is not a Huckel (4n+2) number, so the anion is not aromatic, and the radical has 3 pi electrons.
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