Which of the following cyclic species is NOT aromatic (i.e., not planar and fully conjugated with $4n+2$ $\pi$ electrons)?

• A. Cyclopentadienyl anion ($C_5{H}_5^{-}$)
• B. Cyclobutadiene
• C. Benzene
• D. Pyridine

Answer: (B)

Verified step by step guidance

1. Step 1: Recall the criteria for aromaticity: a molecule must be cyclic, planar, fully conjugated (p orbitals overlapping continuously), and contain a total of \$4n + 2\( π electrons, where \)n$ is a non-negative integer (Hückel's rule).
2. Step 2: Analyze each species for these criteria. First, check if the molecule is cyclic and planar, allowing for continuous overlap of p orbitals (full conjugation).
3. Step 3: Count the number of π electrons in each species. For example, benzene has 6 π electrons, which fits \$4n + 2\( with \)n=1$; the cyclopentadienyl anion has 6 π electrons as well; pyridine is similar to benzene with 6 π electrons.
4. Step 4: Examine cyclobutadiene, which is cyclic and conjugated but has 4 π electrons. This fits the \$4n\( rule (with \)n=1\(), not the \)4n + 2$ rule, indicating antiaromaticity or non-aromaticity due to instability and possible non-planarity.
5. Step 5: Conclude that cyclobutadiene is NOT aromatic because it does not satisfy Hückel's rule and is typically not planar and fully conjugated in its stable form, unlike the other species listed.