Which of the following compound is the most acidic?

p-Nitrophenol

The correct answer is option 3. p-Nitrophenol.

Let us explore the acidity of the compounds in detail, focusing on how the structure and substituents influence their acidity. We'll analyze each compound based on the stability of their conjugate bases, which directly relates to their acidity.

Acidity is the tendency of a compound to donate a proton \((H^+)\). The more stable the conjugate base (the species that remains after the proton is lost), the more acidic the compound.

Conjugate Base Stability: The stability of the conjugate base is affected by factors like resonance, inductive effects, and the presence of electron-withdrawing or electron-donating groups.

1. Benzyl Alcohol

Benzyl alcohol consists of a hydroxyl group \((-OH)\) attached to a benzyl group (a \(-CH_2\) group attached to a benzene ring).

When benzyl alcohol loses a proton, it forms a benzylate ion (\(C_6H_5CH_2O^-\)). The negative charge on the oxygen is not delocalized onto the benzene ring because the -CH₂ group separates the oxygen from the ring, preventing resonance stabilization.

The conjugate base is not stabilized by resonance or other effects, making benzyl alcohol the least acidic among the compounds.

2. Phenol 

Phenol has a hydroxyl group directly attached to the benzene ring. Upon losing a proton, phenol forms a phenoxide ion (\(C_6H_5O^-\)). The negative charge on the oxygen can be delocalized into the aromatic ring through resonance. This delocalization spreads the negative charge over the oxygen and carbon atoms in the ring, stabilizing the conjugate base.

Resonance Structures:

Result: The resonance stabilization of the phenoxide ion makes phenol more acidic than benzyl alcohol.

3. p-Nitrophenol

p-Nitrophenol has a nitro group \((-NO_2)\) at the para position relative to the hydroxyl group on the benzene ring. The nitro group is a strong electron-withdrawing group, and it exerts its effect through both resonance and inductive effects:

The nitro group pulls electron density away from the benzene ring and the oxygen atom, which stabilizes the negative charge on the oxygen in the conjugate base.

The electron-withdrawing nature of the nitro group stabilizes the phenoxide ion further by decreasing the electron density around the oxygen.

Stabilization of the Conjugate Base:

The combined resonance and inductive effects make the conjugate base of p-nitrophenol highly stable, making p-nitrophenol significantly more acidic than phenol.

4. p-Cresol

p-Cresol has a methyl group \((-CH_3)\) at the para position relative to the hydroxyl group. The methyl group is an electron-donating group, which destabilizes the negative charge on the oxygen in the conjugate base through the inductive effect. This increases electron density around the oxygen, making the conjugate base less stable. The electron-donating effect of the methyl group makes p-cresol less acidic than phenol.

Comparison of Acidity

Benzyl Alcohol: Least acidic, as its conjugate base is not resonance-stabilized.

p-Cresol: Less acidic than phenol because the methyl group destabilizes the conjugate base.

Phenol: More acidic than benzyl alcohol and p-cresol due to resonance stabilization of the conjugate base.

p-Nitrophenol: Most acidic because the nitro group significantly stabilizes the conjugate base through resonance and inductive effects.

Conclusion

Most Acidic Compound: p-Nitrophenol (Option 3) is the most acidic among the options provided due to the strong electron-withdrawing effect of the nitro group, which stabilizes the conjugate base more effectively than the other compounds.