Question

The order of acidic strength in the following compounds is A. Phenol
B. p-nitrophenol
C. Ethanol
D. p-methoxyphenol

• A. \( B > A > D > C \)
• B. \( C > B > A > D \)
• C. \( B > D > A > C \)
• D. \( B > A > C > D \)

Answer 1

The Correct Option is A

Concept:
Acidic strength of phenols and alcohols depends on the stability of the conjugate base formed after loss of \(H^+\). Substituents on the aromatic ring influence acidity through inductive and resonance effects.

• Electron-withdrawing groups increase acidity by stabilizing the phenoxide ion.
• Electron-donating groups decrease acidity by destabilizing the phenoxide ion.

Step 1: p-nitrophenol (B) The \(-NO_2\) group is a strong \(-I\) and \(-R\) electron-withdrawing group. It stabilizes the phenoxide ion through resonance and inductive effects. Thus acidity increases significantly. \[ \text{Highest acidity} \] Step 2: Phenol (A) Phenol forms phenoxide ion stabilized by resonance with the aromatic ring. However, it lacks strong electron-withdrawing substituents. Thus its acidity is lower than p-nitrophenol. Step 3: p-methoxyphenol (D) The \(-OCH_3\) group is electron donating due to \(+R\) effect. This increases electron density on the ring and destabilizes the phenoxide ion. Hence acidity decreases compared to phenol. Step 4: Ethanol (C) Alcohols do not have resonance stabilization of the conjugate base. Thus ethanol is the weakest acid among the given compounds. Step 5: Final order of acidity \[ p\text{-nitrophenol} > \text{phenol} > p\text{-methoxyphenol} > \text{ethanol} \] \[ B > A > D > C \] \[ \boxed{B > A > D > C} \]