Question

The compounds pyridine and planar cyclooctatetraene are __________________, respectively

• A. aromatic and non-aromatic
• B. aromatic and anti-aromatic
• C. aromatic and aromatic
• D. anti-aromatic and non-aromatic
• E. anti-aromatic and anti-aromatic

Hint:

To be aromatic, a system must be cyclic, planar, and have 2, 6, 10, 14... $\pi$ electrons. If it is planar and cyclic but has 4, 8, 12... $\pi$ electrons, it is anti-aromatic.

Answer 1

The Correct Option is B

Concept: Hückel's Rule defines aromaticity: cyclic, planar, conjugated systems with $(4n+2)\pi$ electrons are aromatic, while those with $4n\pi$ electrons are anti-aromatic.

Step 1: {Pyridine.} Pyridine is a six-membered heterocyclic ring with three double bonds. It has $6\pi$ electrons ($n=1$), is planar, and cyclic. It satisfies the $(4n+2)$ rule and is aromatic.

Step 2: {Planar Cyclooctatetraene (COT).} COT is an eight-membered ring with four double bonds. The question explicitly states it is "planar." It has $8\pi$ electrons.

Step 3: {Apply Hückel's Rule to COT.} Since 8 is a multiple of 4 ($4n$ where $n=2$), a planar, cyclic, conjugated 8-electron system is anti-aromatic. (Note: Real COT is non-aromatic because it adopts a non-planar tub shape, but the "planar" constraint makes it anti-aromatic).