Question:
Which of the following is most acidic?}
Which of the following is most acidic?}
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Look for electron-withdrawing groups (EWG) like -NO\(_2\), -CHO, -COOH, etc., at ortho or para positions to the -OH group to increase the acidity of phenols.
Updated On: May 2, 2026
- A
- B
- C
- D
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept:
Acidity of phenols is enhanced by electron-withdrawing groups, especially when they are ortho or para to the OH group. This is because these groups stabilize the conjugate base (phenoxide ion) through resonance and inductive effects.
Step 2: Detailed Explanation:
The aldehyde group (\(-CHO\)) is a strong electron-withdrawing group. When it is ortho or para to the OH group, it can participate in resonance with the phenoxide ion, delocalizing the negative charge further onto the oxygen of the aldehyde group, thus stabilizing it. Meta-substitution does not allow this resonance stabilization. A simple alcohol (\(CH_3OH\)) is not acidic.
Step 3: Final Answer:
Therefore, the ortho- or para-hydroxybenzaldehyde is the most acidic among the options.
Acidity of phenols is enhanced by electron-withdrawing groups, especially when they are ortho or para to the OH group. This is because these groups stabilize the conjugate base (phenoxide ion) through resonance and inductive effects.
Step 2: Detailed Explanation:
The aldehyde group (\(-CHO\)) is a strong electron-withdrawing group. When it is ortho or para to the OH group, it can participate in resonance with the phenoxide ion, delocalizing the negative charge further onto the oxygen of the aldehyde group, thus stabilizing it. Meta-substitution does not allow this resonance stabilization. A simple alcohol (\(CH_3OH\)) is not acidic.
Step 3: Final Answer:
Therefore, the ortho- or para-hydroxybenzaldehyde is the most acidic among the options.
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