2,4,6-Trinitrophenol is

Strong Acid

The correct answer is option 1. Strong Acid.

Let us delve deeper into why 2,4,6-Trinitrophenol (picric acid) is classified as a strong acid, including its structure, acidity, and the effects of its substituents.

Structure of 2,4,6-Trinitrophenol

2,4,6-Trinitrophenol is a phenol derivative with three nitro groups attached to the benzene ring. The structure can be visualized as follows:

Phenol (C₆H₅OH): A benzene ring with a hydroxyl group (-OH) attached.

Nitro Groups (-NO₂): Electron-withdrawing groups that are attached to the 2, 4, and 6 positions on the benzene ring.

Acidity of 2,4,6-Trinitrophenol

Electron-Withdrawing Effect of Nitro Groups:

Phenol: In a simple phenol molecule, the -OH group is attached to the benzene ring. The phenol is weakly acidic because the phenoxide ion (the conjugate base) formed after deprotonation is stabilized by resonance. However, phenol is still not very acidic compared to other acids.

2,4,6-Trinitrophenol: The presence of three nitro groups significantly alters the acidity of the phenol:

Nitro Groups: These groups are strongly electron-withdrawing through both inductive and resonance effects. This electron-withdrawing power pulls electron density away from the -OH group and the rest of the benzene ring, making the -OH group more acidic.

Stabilization of Phenoxide Ion: After deprotonation, the negative charge on the oxygen atom in the phenoxide ion (the conjugate base) is significantly stabilized by the electron-withdrawing nitro groups. The nitro groups stabilize the negative charge through resonance and inductive effects, making the phenoxide ion more stable compared to the phenoxide ion from phenol.

Comparison with Other Compounds

Strong Acid: The increased stabilization of the phenoxide ion due to the nitro groups makes 2,4,6-Trinitrophenol (picric acid) a very strong acid. Its acidity is significantly greater than that of phenol, which is a weak acid. Picric acid has a lower pKa value compared to phenol, indicating stronger acidic behavior.

Strong Base / Weak Base / Amphoteric: Picric acid is not a base at all. It does not exhibit basic properties; it is strictly an acid. Amphoteric substances can act as both acids and bases, but picric acid does not show such behavior. It remains acidic due to its structure and the effects of the nitro groups.

Boiling Point and Solubility:

Boiling Point: Picric acid has a high melting point (189°C), reflecting the strength of its acidic nature and its ability to form strong intermolecular interactions due to hydrogen bonding and electrostatic interactions.

Solubility: Picric acid is soluble in water because it can form hydrogen bonds and has polar characteristics due to the nitro groups and the phenolic -OH group

Summary

The strong acidity of 2,4,6-Trinitrophenol is primarily due to the presence of the three nitro groups, which greatly stabilize the phenoxide ion formed after the acid loses a proton. This increased stabilization leads to a lower pKa value and a stronger acidic character compared to phenol.

Thus, 2,4,6-Trinitrophenol is classified as a strong acid due to the electron-withdrawing effects of the nitro groups enhancing the acidity of the phenolic -OH group.