Which of the following heterocycles is not aromatic?

• A. Thiophene
• B. Pyrrole
• C. Furan
• D. Cyclobutadiene

Answer: (D)

Verified step by step guidance

1. Step 1: Recall the criteria for aromaticity. A compound is aromatic if it is cyclic, planar, fully conjugated (p orbitals at every atom in the ring), and follows Hückel's rule, which states it must have \$4n + 2\( π electrons, where \)n$ is a non-negative integer (0, 1, 2, ...).
2. Step 2: Analyze each compound's structure to determine if it meets these criteria. Thiophene, pyrrole, and furan are five-membered heterocycles with a heteroatom contributing lone pair electrons to the π system, making them aromatic.
3. Step 3: Count the π electrons in each heterocycle. For example, in thiophene, the sulfur atom contributes two electrons from its lone pair, and the double bonds contribute four electrons, totaling six π electrons, which fits Hückel's rule (\$4n + 2 = 6\( when \)n=1$). Similar reasoning applies to pyrrole and furan.
4. Step 4: Consider cyclobutadiene, a four-membered ring with two double bonds. It has 4 π electrons, which fits the formula \$4n\( (with \)n=1\(), not \)4n + 2$. This makes it antiaromatic if planar and conjugated, or non-aromatic if it distorts to avoid antiaromaticity.
5. Step 5: Conclude that cyclobutadiene is not aromatic because it does not satisfy Hückel's rule for aromaticity, unlike the other heterocycles listed.