Which of the following compounds is nonaromatic?

• A. Furan
• B. $C_6{H}_6$ (benzene)
• C. Cyclohexene
• D. Pyrrole

Answer: (C)

Verified step by step guidance

1. Recall the criteria for aromaticity: a compound 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. Analyze each compound to check if it meets these criteria. For example, benzene (C\_6H\_6) is a classic aromatic compound because it is cyclic, planar, fully conjugated, and has 6 π electrons (n=1 in Hückel's rule).
3. Examine furan and pyrrole, which are heterocyclic compounds. Both have lone pairs on heteroatoms that contribute to the π system, making them aromatic by satisfying the criteria of planarity, conjugation, and (4n + 2) π electrons.
4. Consider cyclohexene, which is a cyclic compound but not fully conjugated because it contains only one double bond and the rest are single bonds, so it does not have a continuous overlap of p orbitals around the ring.
5. Conclude that cyclohexene is nonaromatic because it fails the full conjugation and Hückel's rule requirements, unlike the other compounds listed.