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Which of the following compound is aromatic?

  • A.Cycloheptatrienyl cation, a seven-membered ring with three double bonds and a positive charge
  • B.Methylenecyclohexadiene, a six-membered ring with two double bonds and an exocyclic =CH2
  • C.Cyclopentadiene, a five-membered ring with two double bonds and one sp3 carbon
  • D.Cyclopentadienyl cation, a five-membered ring with two double bonds and a positive charge
  • E.Cyclooctatetraene, an eight-membered ring with four double bonds

Answer: A

Option A, the cycloheptatrienyl (tropylium) cation, is the aromatic one.

An aromatic species is cyclic, planar and fully conjugated, with \(4n + 2\) pi electrons. In the tropylium cation each of the seven carbons is sp2, the empty p orbital on the charged carbon joins the three double bonds in one ring, and there are 6 pi electrons (\(n = 1\)).

B has an sp3 ring carbon, so the ring is not fully conjugated, and its exocyclic double bond only adds cross-conjugation. C, cyclopentadiene, also has an sp3 carbon. D, the cyclopentadienyl cation, is fully conjugated but has 4 pi electrons, the \(4n\) count that makes it antiaromatic. E, cyclooctatetraene, has 8 pi electrons and puckers into a tub shape, so it is non-planar and non-aromatic.

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