Question

Which of the following lacks aromatic properties?

• A. Fig A
• B. Fig B
• C. Fig C
• D. Fig D

Hint:

\[ \begin{aligned} 6\pi,\;10\pi,\;14\pi &\rightarrow \text{Aromatic} \\ 4\pi,\;8\pi,\;12\pi &\rightarrow \text{Antiaromatic} \end{aligned} \] For aromaticity, the system must be: \[ \boxed{\text{Cyclic + Planar + Conjugated + }(4n+2)\pi} \]

Answer 1

The Correct Option is C

Concept: According to Hückel's rule, a cyclic, planar and completely conjugated system is aromatic if it contains \[ (4n+2)\pi \] electrons. A system containing \[ 4n\pi \] electrons is antiaromatic and therefore lacks aromatic character.

Step 1: Examine option (A). The cyclopentadienyl anion contains \[ 4+2=6\pi \text{ electrons} \] \[ 6=4(1)+2 \] Hence, it satisfies Hückel's rule and is aromatic.

Step 2: Examine option (B). The heterocyclic anion also possesses \[ 6\pi \text{ electrons} \] and satisfies \[ 4n+2 \] electron count. Therefore, it is aromatic.

Step 3: Examine option (C). The cyclooctatetraene-type ring contains \[ 8\pi \text{ electrons} \] \[ 8=4(2) \] Thus it does not satisfy Hückel's rule. Hence, it lacks aromatic character.

Step 4: Examine option (D). The fused polycyclic aromatic hydrocarbon possesses a conjugated aromatic \(\pi\)-electron system and exhibits aromatic character. \[ \begin{aligned} (A) &: \text{Aromatic } (6\pi) \\ (B) &: \text{Aromatic } (6\pi) \\ (C) &: \text{Lacks aromatic character } (8\pi) \\ (D) &: \text{Aromatic} \end{aligned} \] Therefore, \[ \boxed{\text{Option (C) lacks aromatic properties}} \] Hence, option \(\mathbf{(C)}\) is correct.