Which of the following molecules is aromatic? 1. $C_6{H}_6$ (benzene) 2. $C_4{H}_4$ (cyclobutadiene) 3. $C_5{H}_5{-}^{\mathrm{}}$ (cyclopentadienyl anion) 4. $C_7{H}_7$ (tropylium cation)

• A. 1 and 2 are aromatic; 3 and 4 are nonaromatic
• B. 1, 3, and 4 are aromatic; 2 is antiaromatic
• C. Only 1 is aromatic; 2, 3, and 4 are nonaromatic
• D. All four molecules are aromatic

Answer: (B)

Verified step by step guidance

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 states it must have (4n + 2) π electrons, where n is a non-negative integer (0, 1, 2, ...).
2. Analyze benzene (C\_6H\_6): It is a six-membered ring with alternating double bonds, planar, fully conjugated, and has 6 π electrons. Since 6 fits Hückel's rule (4n + 2 = 6 when n=1), benzene is aromatic.
3. Analyze cyclobutadiene (C\_4H\_4): It is a four-membered ring with 4 π electrons. Although it is cyclic and conjugated, 4 π electrons fit the 4n rule (n=1), which makes it antiaromatic if planar. Typically, cyclobutadiene is antiaromatic due to this electron count and planarity.
4. Analyze cyclopentadienyl anion (C\_5H\_5\^-): This five-membered ring has 6 π electrons (4 from double bonds plus 2 from the negative charge). It is cyclic, planar, and fully conjugated. Since 6 π electrons satisfy Hückel's rule, it is aromatic.
5. Analyze tropylium cation (C\_7H\_7\^+): This seven-membered ring cation has 6 π electrons (from three double bonds, with the positive charge removing one electron). It is cyclic, planar, and fully conjugated. With 6 π electrons, it satisfies Hückel's rule and is aromatic.