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Question 36

Which one of the following compounds is non-aromatic?

  • Option A (Furan): Aromatic. The ring is continuous because the oxygen atom contributes one of its lone pairs to the $\pi$ system. This creates a fully conjugated ring with 6 $$\pi$$ electrons, perfectly satisfying the $$4n+2$$ rule (where $$n=1$$).
  • Option B (Anthracene): Aromatic. This molecule is a flat, fully conjugated polycyclic hydrocarbon. It provides a continuous, unbroken path for electron delocalization and contains 14 $$\pi$$ electrons, satisfying the rule (where $$n=3$$).
  • Option C (Cycloocta-1,3,5-triene): Non-aromatic. This is the correct answer because it fails the prerequisite of full conjugation. The two $$sp^3$$ hybridized carbons at the bottom of the ring lack unhybridized p-orbitals, acting as roadblocks that physically break the continuous circuit needed for electron delocalization.
  • Option D (Cyclobutadiene dication): Aromatic. The two positive charges indicate empty p-orbitals on those carbon atoms. These empty orbitals complete the circuit around the ring, allowing the 2 $$\pi$$ electrons from the single double bond to freely delocalize. This satisfies the $$4n+2$$ rule (where $$n=0$$).
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