Question

Which of the following compounds are aromatic?

• A. A, B, C
• B. B, C
• C. B, C, D
• D. A, B, D
• E.

Hint:

To quickly check for a 5-membered ring: - Neutral (Cyclopentadiene): Non-aromatic (due to $sp^3$ carbon). - Anion (Negative charge): 6 \(\pi\) electrons (Aromatic). - Cation (Positive charge): 4 \(\pi\) electrons (Anti-aromatic).

Answer 1

The Correct Option is A

Concept: Aromaticity is defined by Hückel's Rule. For a molecule to be aromatic, it must be cyclic, planar, fully conjugated, and contain \((4n + 2) \, \pi\) electrons (2, 6, 10, 14...).

Step 1: Analysis of Structure A (Pyrrole).

• It contains two \(\pi\) bonds (\(4 \, \pi\) electrons) and a Nitrogen lone pair in a p-orbital.
• Total \(\pi\) electrons = \(4 + 2 = 6\).
• Matches Hückel's number (\(n=1\)), so A is aromatic.

Step 2: Analysis of Structure B (Cyclopentadienyl Anion).

• Two \(\pi\) bonds (\(4 \, \pi\) electrons) plus the lone pair from the negative charge (\(2 \, \pi\) electrons).
• Total \(\pi\) electrons = \(4 + 2 = 6\).
• Matches Hückel's number (\(n=1\)), so B is aromatic.

Step 3: Analysis of Structure C (Cyclopentadienyl Cation).

• The two \(\pi\) bonds contribute \(4 \, \pi\) electrons.
• The positive charge represents an empty p-orbital (0 electrons).
• Total \(\pi\) electrons = 4.
• 4 is a \(4n\) number (Anti-aromatic), not a \((4n+2)\) number. Therefore, C is not aromatic.

Step 4: Analysis of Structure D (Cyclopentadiene).

• The bottom carbon is \(sp^3\) hybridized (attached to two hydrogens).
• This breaks the continuous ring of p-orbitals, so it is not fully conjugated.
• Therefore, D is non-aromatic.