Which of the following compounds is $antiaromatic$?

• A. $Cyclooctatetraene$
• B. $Benzene$
• C. $Cyclopentadienyl$ $-$
• D. $Cyclobutadiene$

Answer: (D)

Verified step by step guidance

1. Recall the definition of antiaromatic compounds: these are cyclic, planar molecules with a continuous overlap of p-orbitals (conjugated system) that contain 4n π electrons, where n is an integer (n = 1, 2, 3, ...). This leads to destabilization due to electron delocalization.
2. Analyze each compound for the criteria of antiaromaticity: it must be cyclic, planar, fully conjugated, and have 4n π electrons.
3. For Cyclooctatetraene, note that it is cyclic and has 8 π electrons, but it adopts a non-planar 'tub' conformation, which prevents continuous overlap of p-orbitals, so it is not antiaromatic.
4. For Benzene, it is cyclic, planar, fully conjugated, and has 6 π electrons (which fits the 4n+2 rule, with n=1), making it aromatic, not antiaromatic.
5. For Cyclopentadienyl anion, it is cyclic, planar, fully conjugated, and has 6 π electrons (4 from double bonds plus 2 from the negative charge), fitting the 4n+2 rule, so it is aromatic, not antiaromatic.
6. For Cyclobutadiene, it is cyclic, planar, fully conjugated, and has 4 π electrons, which fits the 4n rule with n=1, making it antiaromatic due to destabilization from electron delocalization.