Question

Applying the Hückel rule, the number of aromatic compounds among the following is _ _ _. (in integer)

Hint:

Aromatic compounds follow Hückel rule: \[ (4n+2)\pi \] electrons, whereas \(4n\pi\) electron systems are generally antiaromatic if planar and fully conjugated.

Answer 1

Correct Answer: 3

Step 1: Recall Hückel rule.
According to Hückel rule, a compound is aromatic if it is cyclic, planar, completely conjugated, and has
\[ (4n+2)\pi \] electrons, where \(n=0,1,2,3,\ldots\).

Step 2: Analyze cycloheptatrienyl cation.
Cycloheptatrienyl cation has \(6\pi\) electrons.
\[ 6=4(1)+2 \] Therefore, cycloheptatrienyl cation is aromatic.

Step 3: Analyze cyclobutadiene.
Cyclobutadiene has \(4\pi\) electrons.
\[ 4=4(1) \] It follows \(4n\pi\) electron rule, so it is antiaromatic, not aromatic.

Step 4: Analyze cyclooctatetraene.
Cyclooctatetraene has \(8\pi\) electrons. It avoids antiaromaticity by becoming non-planar.
Thus, it is non-aromatic.

Step 5: Analyze cyclopentadienyl anion.
Cyclopentadienyl anion has \(6\pi\) electrons due to two double bonds and one lone pair.
\[ 6=4(1)+2 \] Therefore, it is aromatic.

Step 6: Analyze cyclooctatetraenyl dianion.
Cyclooctatetraenyl dianion has \(10\pi\) electrons.
\[ 10=4(2)+2 \] Therefore, it is aromatic.

Step 7: Count the aromatic compounds.
The aromatic compounds are:
cycloheptatrienyl cation, cyclopentadienyl anion, cyclooctatetraenyl dianion Hence, total number of aromatic compounds is
\[ \boxed{3} \]