Question

Identify the aromatic compounds among the given set based on Huckel's rule:

• A. I, III, IV
• B. I, II, IV
• C. I, II, III
• D. II, III, IV

Hint:

Presence of even one \(sp^3\) carbon in the ring usually breaks aromatic conjugation.

Answer 1

The Correct Option is B

Concept: According to Huckel’s rule, a compound is aromatic if:
• it is cyclic,
• planar,
• fully conjugated,
• contains \((4n+2)\pi\) electrons.

Step 1: Cyclopropenyl cation.
It contains: \[ 2\pi \text{ electrons} \] For: \[ n=0 \] \[ 4n+2 = 2 \] Hence aromatic.

Step 2: Naphthalene.
Naphthalene contains: \[ 10\pi \text{ electrons} \] For: \[ n=2 \] \[ 4(2)+2=10 \] Hence aromatic.

Step 3: Cyclopentadiene.
Cyclopentadiene contains one: \[ sp^3 \] hybridized carbon atom. Therefore conjugation is interrupted. Hence it is non-aromatic.

Step 4: Azulene.
Azulene is fully conjugated and contains: \[ 10\pi \text{ electrons} \] Thus it satisfies Huckel’s rule and is aromatic. Therefore aromatic compounds are: \[ I,\ II,\ IV \]