Which of the following heterocycles is aromatic?

• A. $\mathrm{Pyrrole}$
• B. $\mathrm{Furan}$
• C. $\mathrm{Tetrahydrofuran}$
• D. $\mathrm{Piperidine}$

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 at every atom in the ring), and follow Hückel's rule, which requires having (4n + 2) π electrons, where n is a non-negative integer.
2. Step 2: Analyze each heterocycle to determine if it meets these criteria. Pyrrole and furan are five-membered rings with heteroatoms (nitrogen in pyrrole, oxygen in furan) that contribute lone pair electrons to the π system, making the rings fully conjugated and planar.
3. Step 3: Count the π electrons in the ring for pyrrole and furan. Each has 4 π electrons from the double bonds plus 2 π electrons from the lone pair on the heteroatom, totaling 6 π electrons, which satisfies Hückel's rule (4n + 2, with n=1).
4. Step 4: Examine tetrahydrofuran and piperidine. Both are saturated rings (no double bonds) and their heteroatoms do not contribute lone pairs to a conjugated π system, so they lack the continuous overlap of p orbitals necessary for aromaticity.
5. Step 5: Conclude that pyrrole and furan are aromatic because they meet all aromaticity criteria, while tetrahydrofuran and piperidine are not aromatic due to the absence of conjugation and π electron delocalization.