Question

The compound that produces aromatic species upon deprotonation of the indicated hydrogen is:

• A. 5-methylcyclopentadiene
• B. 3-methylcyclopropene
• C. 7-methylcycloheptatriene
• D. cyclobutane-1,3-dione derivative

Hint:

The cyclopentadienyl anion is one of the classic examples of non-benzenoid aromatic systems. Its conjugate acid (cyclopentadiene) has an unusually low $\text{pK}_a$ ($\approx 16$) for a hydrocarbon because of this aromatic stabilization of the conjugate base.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to identify which of the provided structures becomes aromatic after losing the specific proton ($\text{H}^+$) indicated in the drawing.

Step 2: Key Formula or Approach:
We must apply Huckel's rule of aromaticity to the conjugate bases formed after deprotonation.
According to Huckel's rule, a cyclic, planar, completely conjugated system is aromatic if it contains:
\[ (4n+2)\pi \text{ electrons (where } n = 0, 1, 2, 3, \dots) \]
It is antiaromatic if it contains $4n\pi$ electrons.

Step 3: Detailed Explanation:

• Let us analyze the deprotonation of the indicated $\text{sp}^3$ hybridized carbon in each option.

• In Option A, 5-methylcyclopentadiene contains a CH(Me) group situated between two double bonds.

• Deprotonation at this position removes the proton ($\text{H}^+$), converting the carbon from $\text{sp}^3$ to $\text{sp}^2$ hybridization and leaving behind a lone pair of electrons in a p-orbital.

• This creates the 5-methylcyclopentadienyl anion, which is a planar, monocyclic ring with complete conjugation.

• The total number of $\pi$ electrons in this conjugated loop is:
\[ 2 \text{ (from first C=C)} + 2 \text{ (from second C=C)} + 2 \text{ (from the negative charge/lone pair)} = 6\pi \text{ electrons} \]

• Since $6$ is a Huckel number ($4n+2$ with $n=1$), the resulting anion is highly stable and aromatic.

• In Option B, deprotonation of the cyclopropene derivative yields a cyclopropenyl anion with $4\pi$ electrons, which is antiaromatic and highly unstable.

• In Option C, deprotonation of the cycloheptatriene derivative yields a cycloheptatrienyl anion with $8\pi$ electrons, which is also antiaromatic.

• In Option D, the dicarbonyl derivative contains no continuous conjugated ring system that can form a planar aromatic ring of Huckel size.


Step 4: Final Answer:
Therefore, only the 5-methylcyclopentadiene derivative (Option A) produces an aromatic anion upon deprotonation.