Which one of the following compounds is aromatic (i.e., cyclic, planar, fully conjugated, and containing $4n+2$ $\pi$ electrons)?

• A. Cyclooctatetraene, $C_8{H}_8$
• B. Cyclopentadienyl cation, $C_5{H}_5+$
• C. Cyclopentadienyl anion, $C_5{H}_5-$
• D. Cyclobutadiene, $C_4{H}_4$

Answer: (C)

Verified step by step guidance

1. Step 1: Identify the criteria for aromaticity. A compound must be cyclic, planar, fully conjugated (p orbitals overlapping continuously), and contain a Huckel number of π electrons, which is given by the formula \$4n + 2\(, where \)n$ is a non-negative integer (0, 1, 2, ...).
2. Step 2: Analyze each compound's structure to determine if it is cyclic and planar. For example, cyclooctatetraene is cyclic but adopts a non-planar 'tub' conformation, so it is not aromatic.
3. Step 3: Count the number of π electrons in the conjugated system of each compound. Remember that each double bond contributes 2 π electrons, and charges can add or remove electrons (e.g., an anion adds 2 electrons, a cation removes 2 electrons).
4. Step 4: Check if the number of π electrons fits the \$4n + 2\( rule. For example, the cyclopentadienyl anion has 6 π electrons (from 2 double bonds plus 2 electrons from the negative charge), which fits \)4n + 2\( with \)n=1$.
5. Step 5: Confirm that the compound is fully conjugated and planar to allow continuous overlap of p orbitals. The cyclopentadienyl anion is planar and fully conjugated, satisfying all aromaticity criteria.