Question:medium

Which of the following is the most acidic: Ethanol, Phenol, or \(o\)-Nitrophenol?

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Electron-withdrawing groups such as \(NO_2\) increase acidity by stabilizing the conjugate base through inductive and resonance effects.
Updated On: Apr 30, 2026
  • Ethanol
  • Phenol
  • \(o\)-Nitrophenol
  • All have same acidity
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The Correct Option is C

Solution and Explanation

Step 1: Understanding the Question:
Acidity in organic compounds is determined by the stability of the conjugate base formed after losing a proton (\(H^+\)).
Step 2: Key Formula or Approach:
Acidity \(\propto\) Stability of conjugate base.
Stability is increased by:
1. Resonance (delocalization of negative charge).
2. Electron-Withdrawing Groups (EWG) via \(-I\) and \(-M\) effects.
Step 3: Detailed Explanation:
1. Ethanol: The conjugate base is the ethoxide ion (\(CH_3CH_2O^-\)). The alkyl group has a \(+I\) effect, which destabilizes the negative charge. Hence, it is the least acidic.
2. Phenol: The conjugate base is the phenoxide ion. The negative charge is delocalized over the benzene ring via resonance, making it much more stable than ethoxide.
3. \(o\)-Nitrophenol: Here, the nitro group (\(-NO_2\)) is present at the ortho position. It is a strong EWG that exerts both \(-I\) and \(-M\) effects. This further stabilizes the phenoxide ion by pulling electron density away from the oxygen.
Conclusion: \(o\)-Nitrophenol forms the most stable conjugate base.
Step 4: Final Answer:
\(o\)-Nitrophenol is the most acidic among the choices.
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