Step 1: Understanding the Concept:
The acidic strength of an organic compound depends on its ability to lose a proton ($\text{H}^+$) easily and form a stable conjugate base. For substituted phenols, losing a proton leaves behind a negative charge on the oxygen atom, forming a phenoxide ion. Substituents attached to the benzene ring alter the stability of this phenoxide ion via inductive effects ($\pm\text{I}$) and resonance/mesomeric effects ($\pm\text{M}$):
- Electron-Withdrawing Groups (EWGs): Stabilize the negative charge on the phenoxide ion by pulling electron density away, which increases the acidic strength.
- Electron-Donating Groups (EDGs): Destabilize the phenoxide ion by pushing additional electron density toward it, which decreases the acidic strength.
Step 2: Detailed Explanation:
Let's analyze the electrical electronic effects of each substituent at the para-position:
1. p-Nitrophenol: The nitro group ($-\text{NO}_2$) is a powerful Electron-Withdrawing Group. It exerts a strong negative inductive effect ($-\text{I}$) and a negative resonance/mesomeric effect ($-\text{M}$). This pulls electron density away from the phenoxide oxygen atom and strongly delocalizes the negative charge throughout the aromatic ring. This stabilization makes it highly favorable to release the proton, making p-nitrophenol the most acidic compound in this group.
2. Phenol: Plain phenol contains no substituents on its ring. It acts as our baseline reference point. Its phenoxide ion is stabilized solely by the normal resonance of the unsubstituted aromatic ring.
3. p-Methoxyphenol: The methoxy group ($-\text{OCH}_3$) contains unshared lone pairs of electrons on its oxygen atom. At the para-position, it acts as an Electron-Donating Group because its strong positive resonance effect ($+\text{M}$) outweighs its minor negative inductive effect ($-\text{I}$). This electron donation pushes additional negative charge toward the phenoxide oxygen, destabilizing the conjugate base. This makes it difficult to release the proton, rendering p-methoxyphenol the least acidic compound in this group.
Arranging these compounds from strongest acid to weakest acid yields the sequence: p-Nitrophenol > Phenol > p-Methoxyphenol. This tracks with option (B).
Step 3: Final Answer:
The correct order of acidic strength is p-Nitrophenol > Phenol > p-Methoxyphenol.