Which of the following compounds is classified as an aromatic hydrocarbon?

• A. Cyclopentane ($C_5{H}_{10}$)
• B. Benzene ($C_6{H}_6$)
• C. Cyclohexane ($C_6{H}_{12}$)
• D. Cyclobutane ($C_4{H}_8$)

Answer: (B)

Verified step by step guidance

1. Recall the definition of aromatic hydrocarbons: they are cyclic, planar molecules with a conjugated system of \( \pi \)-electrons that follow Hückel's rule, which states the molecule must have \( 4n + 2 \) \( \pi \)-electrons (where \( n \) is a non-negative integer).
2. Examine each compound's structure to determine if it is cyclic and planar, and if it has a conjugated \( \pi \)-electron system. For example, cyclopentane, cyclohexane, and cyclobutane are saturated cyclic hydrocarbons without conjugated double bonds, so they lack \( \pi \)-electrons.
3. Identify benzene's structure: it is a six-membered ring with alternating double bonds, making it fully conjugated and planar, with 6 \( \pi \)-electrons, which fits Hückel's rule (\( 4n + 2 = 6 \) when \( n = 1 \)).
4. Confirm that benzene's \( \pi \)-electron system is delocalized over the ring, providing extra stability characteristic of aromatic compounds.
5. Conclude that among the given options, only benzene meets all criteria for aromaticity and is therefore classified as an aromatic hydrocarbon.