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: State Huckel's rule.
A ring is aromatic if it is flat, fully conjugated, and holds $(4n+2)\pi$ electrons, that is $2, 6, 10, \ldots$ electrons.

Step 2: Check the six electron rings.
The cycloheptatrienyl cation has $6\pi$ electrons and the cyclopentadienyl anion also has $6\pi$ electrons. Both fit $4n+2$, so both are aromatic.

Step 3: Check the cyclooctatetraenyl dianion.
Adding two electrons to cyclooctatetraene gives $10\pi$ electrons, which is $4(2)+2$, so this dianion is aromatic and flattens out to suit it.

Step 4: Reject the rest.
Cyclobutadiene has $4\pi$ electrons, so it is antiaromatic. Neutral cyclooctatetraene has $8\pi$ electrons and puckers to stay non-aromatic. That leaves three aromatic species.

Step 5: Answer.
\[ \boxed{3} \]