Organic Chemistry 2 · Aromaticity and cyclic conjugation
A missing orbital in a five-membered ring
Conditions: Neutral cyclopentadiene; the carbon between the two terminal alkene positions is CH₂.
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The question
Does neutral cyclopentadiene qualify as antiaromatic because it has four π electrons?
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Answer and explanation
Correct answer No; the saturated CH₂ interrupts the cyclic p-orbital path.
Four electrons are insufficient to invoke antiaromaticity without continuous conjugation.
Not this one Yes; any ring containing two double bonds is antiaromatic.
The orbital-continuity requirement is missing.
Not this one No; every five-membered ring has six π electrons.
Ring size does not determine electron count.
Worked solution
Two double bonds provide four π electrons, but the fifth carbon is tetrahedral and has no participating p orbital. Classify the ordinary molecule as nonaromatic, not as a planar conjugated four-electron circuit.
What you'll practice
Distinguish nonaromaticity from antiaromaticity when conjugation is interrupted.
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