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

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.