Question

Arrange the following in increasing order of their pH values:
(A) p-Nitrophenol,
(B) m-Cresol,
(C) m-Nitrophenol,
(D) Phenol.

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

Answer 1

The Correct Option is D

In order to arrange the given phenolic compounds in increasing order of their pH values, we need to understand their acidic strength. The pH value is inversely related to acidity; a stronger acid will have a lower pH. Thus, we need to evaluate their acidic strengths:

• p-Nitrophenol (A): The presence of the electron-withdrawing nitro group increases acidity by stabilizing the phenoxide ion through resonance and inductive effects.
• m-Nitrophenol (C): Similar to p-nitrophenol, the nitro group in meta position is moderately electron-withdrawing but less effective in resonance stabilization compared to the para position.
• m-Cresol (B): The methyl group is electron-donating, reducing acidic strength compared to phenol due to increased electron density destabilizing the phenoxide ion.
• Phenol (D): It is a weak acid where the hydroxyl group contributes slightly to acidity through resonance stabilization.

Considering these effects, the order of acidic strength from strongest to weakest is:

1. m-Nitrophenol (C): Strongest due to electron-withdrawing nitro group, resulting in the lowest pH.
2. m-Cresol (B): Weaker than phenol but stronger acid compared to electron-donating para position.
3. Phenol (D): Lower acidity compared to nitrophenols, higher than cresol due to minimal electron donation.
4. p-Nitrophenol (A): Due to its lowest oxidation state and position of nitro group weakening, it possesses the highest pH.

Thus, the increasing order of their pH values is:

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

Answer 2

The pH of phenolic compounds is affected by the nature and position of substituents on the aromatic ring. Electron-withdrawing groups (like nitro groups) tend to decrease the pH (make the compound more acidic), while electron-donating groups (like methyl groups) increase the pH (make the compound less acidic).

• p-Nitrophenol (A): The nitro group at the para position is a strong electron-withdrawing group, which increases the acidity of the phenol, resulting in a lower pH.
• m-Cresol (B): The methyl group is an electron-donating group, which makes the phenol less acidic compared to p-nitrophenol. Thus, m-cresol has a higher pH than p-nitrophenol.
• m-Nitrophenol (C): The nitro group at the meta position is still electron-withdrawing, but it is less effective in deactivating the phenol than the nitro group at the para position. Therefore, m-nitrophenol is slightly less acidic than p-nitrophenol and has a higher pH.
• Phenol (D): As a neutral compound, phenol has a pH higher than m-nitrophenol, but lower than m-cresol, due to no electron-donating or electron-withdrawing groups influencing the acidity significantly.

Thus, the increasing order of pH values is: (C) < (B) < (D) < (A), which corresponds to Option 4.