Which of the following heterocycles is $\mathrm{aromatic}$?

• A. $\mathrm{Pyrrole}$
• B. $\mathrm{Cyclobutane}$
• C. $\mathrm{Cyclohexanone}$
• D. $\mathrm{Piperidine}$

Answer: (A)

Verified step by step guidance

1. Step 1: Recall the criteria for aromaticity. A compound is aromatic if it is cyclic, planar, fully conjugated (p orbitals at every atom in the ring), and follows Hückel's rule, which requires having (4n + 2) π electrons, where n is a non-negative integer.
2. Step 2: Analyze each compound to see if it meets these criteria. Cyclobutane is a four-membered ring with only sigma bonds and no conjugation, so it is not aromatic.
3. Step 3: Cyclohexanone is a six-membered ring with a ketone group; it is saturated except for the carbonyl, and it does not have a continuous conjugated π system around the ring, so it is not aromatic.
4. Step 4: Piperidine is a six-membered saturated ring containing nitrogen, but it has only sigma bonds and no conjugated π system, so it is not aromatic.
5. Step 5: Pyrrole is a five-membered ring with one nitrogen atom and two double bonds. The nitrogen contributes a lone pair to the conjugated system, making the ring fully conjugated and planar. Counting the π electrons (4 from the double bonds + 2 from the nitrogen lone pair) gives 6 π electrons, which satisfies Hückel's rule, confirming that pyrrole is aromatic.