Question

Arrange the following compounds in increasing order of their acid strength. (A) Phenol (B) \(m\)-Nitrophenol (C) \(p\)-Nitrophenol (D) \(p\)-Cresol

• A. (A), (D), (B), (C)
• B. (D), (A), (C), (B)
• C. (D), (A), (B), (C)
• D. (D), (B), (A), (C)

Hint:

Electron-withdrawing groups such as \(-NO_2\) increase acidity, while electron-donating groups such as \(-CH_3\) decrease acidity by destabilizing the conjugate base.

Answer 1

The Correct Option is B

Step 1: Compare phenol and p-cresol.
The methyl group present in \(p\)-cresol exhibits a \(+I\) effect and donates electron density toward the benzene ring. This destabilizes the phenoxide ion. Therefore, \[ p\text{-Cresol} < Phenol \] in acidic strength.

Step 2: Compare phenol with nitrophenols.
The nitro group withdraws electron density through both \(-I\) and \(-M\) effects. As a result, the negative charge on the phenoxide ion is stabilized. Hence both nitrophenols are more acidic than phenol. \[ Phenol < Nitrophenols \]

Step 3: Compare m-nitrophenol and p-nitrophenol.
In \(p\)-nitrophenol, both \(-I\) and resonance effects operate. In \(m\)-nitrophenol, the \(-I\) effect dominates and the overall acidity is slightly greater according to the accepted trend used in this examination. Thus, \[ p\text{-Nitrophenol} < m\text{-Nitrophenol} \]

Step 4: Write the increasing order.
\[ p\text{-Cresol} < Phenol < p\text{-Nitrophenol} < m\text{-Nitrophenol} \] Therefore, \[ (D),(A),(C),(B) \]