JEE Chemistry 2024 Question with Solution

Given: The question asks which structure has a highly acidic hydrogen.

Find: The correct option containing the most acidic hydrogen.

From the solution, the correct option is D.

The key idea is to compare how stable the conjugate base becomes after removal of a proton. A hydrogen is highly acidic when its removal gives a strongly resonance-stabilized anion.

For structure $4$, the methylene group $\text{CH}_2$ lies between two carbonyl groups $\text{C}=\text{O}$. On removal of a hydrogen, the resulting negative charge is delocalized over both carbonyl groups by resonance.

Because the conjugate base is much more stable, the hydrogen in structure $4$ is highly acidic.

Therefore, the correct option is D.

Given: Acidic hydrogen depends on the stability of the conjugate base formed after deprotonation.

Find: Why structure $4$ has highly acidic hydrogen.

In structure $4$, the hydrogen is present on a methylene carbon located between two carbonyl groups. After loss of $\text{H}^+$, the carbanion formed is resonance stabilized.

The negative charge does not remain localized on one carbon atom. It is delocalized toward both adjacent carbonyl groups, which greatly stabilizes the conjugate base.

This is why hydrogen on an active methylene group between two $\text{C}=\text{O}$ groups is much more acidic than ordinary alkane hydrogens and more acidic than many other common functional group hydrogens.

Hence, structure $4$ contains the highly acidic hydrogen, so the correct option is D.