Benzene ring in phenol is expected to give the characteristic substitution or replacement reactions, i.e., halogenation, nitration, sulphonation, alkylation, etc. In these reactions, the attacking reagent is electrophile. The phenolic group activates the ortho and para positions in the ring due to conjugation. Disubstitution reactions are quite fast as the electron density in the ring has increased considerably. Phenols do not respond to nucleophilic substitution reactions as it is influenced by phenolic group.

Picric acid is a very strong acid due to the presence of:

Three \(NO_2\) groups

The answer is (2) Three \(NO_2\) groups.

Picric acid, also known as 2,4,6-trinitrophenol,  is a strong acid due to the presence of three nitro \((-NO_2)\) groups. The nitro group is a powerful electron-withdrawing group, and its presence on a molecule can increase the acidity of the molecule. In the case of picric acid, the three nitro groups stabilize the conjugate base of picric acid, the picrate ion. This stabilization is due to the resonance of the nitro groups, which allows the negative charge on the oxygen atom of the hydroxyl group to be delocalized over the entire molecule. As a result, the picrate ion is more stable than the hydroxide ion, and picric acid is a stronger acid than water.

Here is a more detailed explanation of the factors that contribute to the acidity of picric acid:

1. Electron withdrawal by the nitro groups: The nitro group is a powerful electron-withdrawing group, and its presence on a molecule can increase the acidity of the molecule. This is because the nitro group pulls electrons away from the adjacent oxygen atom, making the oxygen atom more electronegative and more likely to lose a proton \((H^+)\).
2. Resonance of the nitro groups: The nitro group can also stabilize the conjugate base of picric acid, the picrate ion, through resonance. Resonance is a phenomenon in which a molecule can exist in two or more equivalent Lewis structures. In the case of the picrate ion, the negative charge on the oxygen atom can be delocalized over the entire molecule, making the ion more stable.
3. Induction effect: The nitro group can also stabilize the conjugate base of picric acid through the induction effect. The induction effect is a phenomenon in which a substituent group can influence the electron density of an adjacent atom through the sigma bond. In the case of the picrate ion, the nitro group pulls electrons away from the adjacent oxygen atom, making the oxygen atom more acidic.

As a result of these factors, picric acid is a very strong acid with a pKa of 0.4. This means that picric acid is about 10 million times more acidic than water. Picric acid is used in a variety of applications, including explosives, dyes, and pharmaceuticals.