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. $C_7{H}_7{+}^{\mathrm{}}$ (tropylium ion)

• A. Only 2 and 3 are aromatic
• B. Only 1 and 2 are aromatic
• C. 1, 3, and 4 are aromatic
• D. All four are aromatic

Answer: (C)

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.
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 two double bonds, but it has 4 π electrons. 4 does not satisfy Hückel's rule (4n + 2), and cyclobutadiene is known to be antiaromatic or non-aromatic due to instability and lack of aromatic stabilization.
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 planar and fully conjugated, and 6 π electrons satisfy Hückel's rule, so it is aromatic.
5. Analyze tropylium ion (C\_7H\_7\^+): This seven-membered ring cation has 6 π electrons (from three double bonds), is planar and fully conjugated. With 6 π electrons, it satisfies Hückel's rule and is aromatic.