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Which of the following cyclic conjugated molecules, in pure form, is (are) unstable (highly reactive) at room temperature due to antiaromaticity? Options: cyclohexadiene, cyclobutadiene, cyclopentadienone, and a tropone-like seven-membered ring ketone.

1. Cyclohexadiene
2. Cyclobutadiene
3. Cyclopentadienone
4. Tropone (seven-membered ring ketone)

Correct answer: Cyclobutadiene

Solution

Cyclobutadiene is a planar, fully conjugated 4 pi-electron ring (4n, n=1), which is antiaromatic and extremely unstable, existing only at very low temperatures or trapped. Cyclohexadiene is not fully conjugated/aromatic and is a stable diene. Tropone is stabilized (its resonance form gives an aromatic tropylium-like 6 pi cation oxide) and is reasonably stable. Cyclopentadienone is reactive (tends to dimerize) but the classic answer for inherent instability due to antiaromaticity is cyclobutadiene. Hence cyclobutadiene is the unstable species.

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