Question

Which one of the following compounds is not aromatic ? 

• A. A
• B. B
• C. C
• D. D

Hint:

Remember the three classes:
\textbf{Aromatic:} Cyclic, planar, conjugated, (4n+2) \(\pi\) e\(^-\). Very stable.
\textbf{Anti-aromatic:} Cyclic, planar, conjugated, 4n \(\pi\) e\(^-\). Very unstable.
\textbf{Non-aromatic:} Fails any of the first three conditions (not cyclic, not planar, or not conjugated). Stability is comparable to a similar open-chain compound.
Cyclooctatetraene is a key example of a molecule that puckers to avoid anti-aromaticity.

Answer 1

The Correct Option is B

Step 1: Understanding the Question
We need to identify the non-aromatic compound from the given options. To do this, we apply Hückel's rules for aromaticity.
Step 2: Hückel's Rules for Aromaticity
A compound is considered aromatic if it meets all the following criteria:
1. It must be cyclic.
2. It must be planar.
3. It must be fully conjugated (have a continuous ring of p-orbitals).
4. It must contain (4n + 2) \(\pi\) electrons, where 'n' is a non-negative integer (0, 1, 2, ...).
A compound that satisfies the first three conditions but has 4n \(\pi\) electrons is anti-aromatic. A compound that fails any of the first three conditions is non-aromatic.
Step 3: Analyzing Each Option
(A) Cyclopentadienyl anion:
It is cyclic, planar, and fully conjugated. It has 2 \(\pi\) bonds (4 \(\pi\) electrons) + 1 lone pair on the carbon with a negative charge (2 \(\pi\) electrons) = 6 \(\pi\) electrons. 6 fits the (4n + 2) rule for n=1 (4(1)+2 = 6). Hence, it is aromatic.
(B) Cyclooctatetraene:
It is cyclic and has 4 \(\pi\) bonds, which means 8 \(\pi\) electrons. 8 fits the 4n rule for n=2. If it were planar, it would be anti-aromatic and highly unstable. To avoid this instability, the molecule adopts a non-planar, tub-like shape. Since it is not planar, it is classified as non-aromatic.
(C) Tropylium cation:
It is a seven-membered ring with a positive charge. It is cyclic, planar, and fully conjugated. It has 3 \(\pi\) bonds = 6 \(\pi\) electrons. 6 fits the (4n + 2) rule for n=1. Hence, it is aromatic.
(D) Phenol:
Phenol contains a benzene ring. The benzene ring is cyclic, planar, fully conjugated, and has 6 \(\pi\) electrons. It is a classic example of an aromatic compound.
Step 4: Final Answer
Cyclooctatetraene is not aromatic because it is a non-planar molecule.