Which of the following species is aromatic?

• A. $C_7{H}_8$ (tropylium cation)
• B. $C_6{H}_6$ (benzene)
• C. $C_4{H}_8$ (cyclobutane)
• D. $C_5{H}_5{-}^{\mathrm{}}$ (cyclopentadienyl anion)

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.
2. Analyze each species to determine if it meets these criteria. First, check if the molecule is cyclic and planar, allowing for continuous overlap of p orbitals.
3. Count the number of π electrons in the ring system of each species. For example, benzene has 6 π electrons from three double bonds, which fits Hückel's rule with n=1.
4. Evaluate the tropylium cation (C\_7H\_7\^+), which is a 7-membered ring with a positive charge. Determine if it has (4n + 2) π electrons and if it is planar and fully conjugated.
5. For cyclobutane (C\_4H\_8), check if it is planar and conjugated; typically, cyclobutane is not aromatic because it is not planar and lacks conjugation. For the cyclopentadienyl anion (C\_5H\_5\^-), count π electrons and check planarity and conjugation to see if it fits Hückel's rule.