The aromatic parent hydrocarbon is shown for comparison. Select all of the planar species shown that are antiaromatic.

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A, B, C

Antiaromatic planar rings have 4n pi electrons. The cyclopentadienyl cation (4), cyclobutadiene (4) and the cycloheptatrienyl anion (8) qualify. The tropylium cation has 6 pi electrons and is aromatic.

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