Ordering Compounds by Acidic Strength

This solution explains how to determine the increasing order of acidic strength for phenol and various nitrophenols. Acidic strength is determined by the stability of the conjugate base formed after donating a proton ($H^+$).

Understanding Acidic Strength and Phenoxide Stability

A compound is considered acidic if it can donate a proton. The strength of an acid is related to how stable its conjugate base is. For phenols, the conjugate base is the phenoxide ion ($Ph-O^-$). Electron-withdrawing groups (EWGs) stabilize the negative charge on the oxygen atom, making the corresponding phenol more acidic. Conversely, electron-donating groups (EDGs) destabilize the phenoxide ion, reducing acidity.

Nitro groups ($-\text{NO}_2$) are strong EWGs, primarily acting through inductive effect ($-\text{I}$) and resonance effect ($-\text{R}$). Their position relative to the hydroxyl group ($-\text{OH}$) significantly impacts acidity.

Analysis of Each Compound

• (D) Phenol: This is the basic structure without any substituent groups. Its acidity is the reference point. The phenoxide ion is stabilized by the benzene ring through resonance, but less so than substituted phenols.
• (A) 3-nitrophenol: The nitro group is at the meta position. It withdraws electron density primarily through the inductive effect ($-\text{I}$ effect). This stabilizes the phenoxide ion compared to phenol, increasing acidity. The resonance effect ($-\text{R}$ effect) is minimal at the meta position for stabilizing the negative charge on the oxygen.
• (B) 3,5-Dinitrophenol: This compound has two nitro groups at meta positions. The combined inductive effect ($-\text{I}$) of both groups withdraws more electron density, further stabilizing the phenoxide ion. Therefore, it is more acidic than 3-nitrophenol.
• (C) 2,4,6-Trinitrophenol (Picric Acid): This compound has three nitro groups.
  • The nitro groups at the ortho ($2$ and $6$) and para ($4$) positions strongly withdraw electron density through both inductive ($-\text{I}$) and resonance ($-\text{R}$) effects.
  • The ortho and para positions allow the negative charge of the phenoxide ion to be delocalized onto the oxygen atoms of the nitro groups via resonance. This provides significant stabilization.
  • Additionally, intramolecular hydrogen bonding between the $-\text{OH}$ proton and the ortho nitro group stabilizes the acid molecule itself, making proton donation easier.
  Due to the cumulative effect of three potent EWGs, especially at ortho/para positions, 2,4,6-trinitrophenol is significantly more acidic than the others.

Determining the Order of Acidity

Based on the effects of the nitro groups:

1. Phenol (D) is the least acidic.
2. 3-nitrophenol (A) is more acidic than phenol due to the meta $-\text{NO}_2$ group's inductive effect.
3. 3,5-Dinitrophenol (B) is more acidic than 3-nitrophenol (A) due to the presence of two meta $-\text{NO}_2$ groups.
4. 2,4,6-Trinitrophenol (C) is the most acidic due to the strong electron-withdrawing effects (inductive and resonance) of the three $-\text{NO}_2$ groups, particularly those at the ortho and para positions.

Final Arrangement

The increasing order of acidic strength is:

Phenol < 3-nitrophenol < 3,5-Dinitrophenol < 2,4,6-Trinitrophenol

Represented by the labels:

(D) < (A) < (B) < (C)

Therefore, the correct order is (D), (A), (B), (C).