Question: Choose the compound which is more acidic than phenol.

Step 1 (Governing Principle & Data Analysis): [1/3 Mark]

The acidity of phenols is determined by the stability of the phenoxide ion formed after the loss of a proton ($H^+$). Electron-withdrawing groups (EWG) stabilize the phenoxide ion through $-I$ (inductive) and $-M$ (mesomeric) effects, thereby increasing acidity. Conversely, electron-donating groups (EDG) destabilize the phenoxide ion, decreasing acidity.

• Phenol: Reference compound.
• (a) o-nitrophenol: Contains $-NO_2$ group (strong $-I$ and $-M$ effect).
• (b) Ethanol: Aliphatic alcohol (less acidic than phenol).
• (c) o-methylphenol: Contains $-CH_3$ group ($+I$ and hyperconjugation effect).
• (d) o-methoxyphenol: Contains $-OCH_3$ group ($+M$ effect dominates).

Step 2 (Step-by-Step Comparative Analysis): [1/3 Mark]

We compare the substituent effects on the phenoxide ion stability:

• For o-nitrophenol, the $-NO_2$ group is a strong electron-withdrawing group. It disperses the negative charge on the oxygen atom of the phenoxide ion via the resonance effect ($-M$) and inductive effect ($-I$), significantly increasing the stability of the conjugate base.
• Mathematically, the acidity constant ($K_a$) for phenol is approximately $1.3 \times 10^{-10}$, whereas for o-nitrophenol, it is approximately $6.0 \times 10^{-8}$. Since $pK_a = -\log K_a$, a higher $K_a$ value indicates higher acidity.
• $$K_a(\text{o-nitrophenol}) > K_a(\text{phenol})$$

Step 3 (Final Concluded Answer): [1/3 Mark]

Based on the electronic effects, o-nitrophenol is more acidic than phenol due to the strong electron-withdrawing nature of the nitro group.

Correct Option: (a) o-nitrophenol

💡 EL10 Concept Insight: Think of the phenoxide ion as a "hot potato" (the negative charge). An electron-withdrawing group like $-NO_2$ acts like a cooling fan that pulls the charge away, making the ion stable and the acid stronger, while electron-donating groups just make the "hot potato" even hotter and less stable.