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JEE Main · 2021 · 26 Aug 2021 · Shift 2 · General Organic Chemistry

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Which one of the following compounds is not aromatic?
  1. (A)(see structure)
  2. (B)(see structure)
  3. (C)(see structure)
    ✓ Correct
  4. (D)(see structure)

Answer: (C)

Solution

A ring is aromatic when it is cyclic, planar, fully conjugated and holds electrons. Phenol has a benzene ring with six electrons, the tropylium cation spreads six electrons over seven carbons, and the cyclopentadienyl anion spreads six electrons over five carbons, so all three obey Huckel's rule. Cyclooctatetraene has eight electrons, a count, and escapes the destabilisation of a planar antiaromatic ring by puckering into a tub shape in which neighbouring double bonds are no longer parallel and conjugation is lost. The compound that is not aromatic is cyclooctatetraene.

JEE Main · 2021 · 26 Aug 2021 · Shift 2 · Single correct option

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