Discussion
The non-aromatic compound/ion is
- A.
- B.
- C.
- D.
Answer: A
The non-aromatic species is (A), the ten-membered ring cyclodecapentaene ([10]annulene).
It has 10 pi electrons, which satisfies the 4n + 2 rule for n = 2, but a planar all-cis ten-membered ring would have very strained bond angles, and the isomer drawn would put two inner hydrogens on top of each other. The ring therefore twists out of plane, the p orbitals cannot overlap all round, and the compound behaves as a non-aromatic polyene.
The cycloheptatrienyl (tropylium) cation (B) has 6 pi electrons in a planar ring and the cyclooctatetraene dianion (C) has 10; both are aromatic. The cyclopentadienyl cation (D) has 4 pi electrons in a planar ring and is antiaromatic, which is different from non-aromatic: it is destabilised by cyclic conjugation rather than simply lacking it.
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