Question

Choose the compound which is more acidic than phenol:

• A. \( o\text{-}nitrophenol \)
• B. ethanol
• C. \( o\text{-}methylphenol \)
• D. \( o\text{-}methoxyphenol \)

Hint:

Remember: Electron-withdrawing groups like \(-NO_2\) increase acidity, while electron-donating groups like \(-CH_3\) or \(-OCH_3\) decrease it.

Answer 1

The Correct Option is A

Step 1: Phenol is weakly acidic because the phenoxide ion formed after deprotonation is stabilized by resonance.
Step 2: An electron-withdrawing group increases acidity by stabilizing the negative charge on the oxygen atom.
The nitro group (\(-NO_2\)) is a strong electron-withdrawing group due to its \(-M\) (mesomeric) and \(-I\) (inductive) effects.
Step 3: In \( o\text{-}nitrophenol \), the nitro group is ortho to the hydroxyl group, so it withdraws electron density both through the ring and by induction.
This additional stabilization makes the conjugate base (phenoxide ion) more stable, thereby increasing acidity compared to phenol.
Step 4: Ethanol is less acidic than phenol because it lacks resonance stabilization of the alkoxide ion.
Methyl and methoxy groups are electron-donating due to their \( +I \) and \( +M \) effects, which increase electron density and thus reduce acidity relative to phenol.