Which of the following compounds is aromatic?

• A. $C_6{H}_6$ (benzene)
• B. $C_4{H}_8$ (cyclobutene)
• C. $C_5{H}_{10}$ (cyclopentene)
• D. $C_6{H}_{12}$ (cyclohexane)

Answer: (A)

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 states it must have (4n + 2) π electrons, where n is a non-negative integer.
2. Examine each compound to determine if it is cyclic and planar. Benzene (C\6H\6) is a well-known planar cyclic compound. Cyclobutene (C\4H\8), cyclopentene (C\5H\10), and cyclohexane (C\6H\12) are also cyclic, but their planarity and conjugation need to be assessed.
3. Check for conjugation and count the number of π electrons in each compound. Benzene has 6 π electrons from three double bonds, which are fully conjugated. Cyclobutene and cyclopentene each have one double bond, so fewer π electrons and limited conjugation. Cyclohexane has no double bonds, so no π electrons.
4. Apply Hückel's rule to the π electron count: for aromaticity, the number of π electrons must satisfy 4n + 2. Benzene has 6 π electrons, which fits the rule with n=1. The other compounds do not have the correct number of π electrons or conjugation to be aromatic.
5. Conclude that benzene is aromatic because it meets all the criteria: cyclic, planar, fully conjugated, and has 6 π electrons following Hückel's rule. The other compounds are not aromatic.