In the context of Hückel’s rule, which of the following species is aromatic?

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

Answer: (A)

Verified step by step guidance

1. Recall Hückel's rule for aromaticity: a planar, cyclic, fully conjugated molecule is aromatic if it contains \$4n + 2\( π electrons, where \)n$ is a non-negative integer (0, 1, 2, ...).
2. Determine the number of π electrons in each species by counting the electrons in the conjugated p orbitals. Remember that each double bond contributes 2 π electrons, and charges can add or remove electrons from the system.
3. For the cyclopentadienyl anion (\(\mathrm{C_5H_5^-}\)), count the π electrons: the ring has 2 double bonds (4 π electrons) plus 2 electrons from the negative charge (a lone pair), totaling 6 π electrons.
4. For the cyclopentadienyl cation (\(\mathrm{C_5H_5^+}\)), count the π electrons: the ring has 2 double bonds (4 π electrons) but loses one electron due to the positive charge, resulting in 4 π electrons.
5. Apply Hückel's rule to each: 6 π electrons fit \$4n + 2\( with \)n=1\(, indicating aromaticity; 4 π electrons fit \)4n\( (with \)n=1$), which corresponds to antiaromatic or non-aromatic species. Repeat this analysis for cyclooctatetraene and cyclobutadiene considering their electron counts and molecular geometry.