Question

Among the following compounds, the correct order of acidic strength is: \[ \text{Phenol},\quad p\text{-Nitrophenol},\quad p\text{-Methoxyphenol} \]

• A. \(p\text{-Methoxyphenol} > \text{Phenol} > p\text{-Nitrophenol}\)
• B. \(p\text{-Nitrophenol} > \text{Phenol} > p\text{-Methoxyphenol}\)
• C. \(\text{Phenol} > p\text{-Nitrophenol} > p\text{-Methoxyphenol}\)
• D. \(p\text{-Nitrophenol} > p\text{-Methoxyphenol} > \text{Phenol}\)

Hint:

Electron withdrawing groups increase acidity by stabilizing conjugate base, while electron donating groups decrease acidity by destabilizing the conjugate base.

Answer 1

The Correct Option is B

Concept: Acidic strength depends upon stability of conjugate base.
• Electron withdrawing groups increase acidity
• Electron donating groups decrease acidity Greater stabilization of phenoxide ion means stronger acid.

Step 1: Analyse \(p\)-nitrophenol. Nitro group: \[ (-NO_2) \] is strongly electron withdrawing. It stabilizes phenoxide ion by:
• \(-I\) effect
• \(-R\) effect Thus acidity increases significantly. Therefore: \[ p\text{-Nitrophenol is most acidic} \]

Step 2: Analyse phenol. Phenol has no strong substituent. Hence its acidity is intermediate.

Step 3: Analyse \(p\)-methoxyphenol. Methoxy group: \[ (-OCH_3) \] is electron donating through resonance. It destabilizes phenoxide ion. Thus acidity decreases. Therefore: \[ p\text{-Methoxyphenol is least acidic} \]

Step 4: Write the correct order. Hence: \[ p\text{-Nitrophenol} > \text{Phenol} > p\text{-Methoxyphenol} \] Thus the correct option is: \[ \boxed{(B)} \]