Question

The correct acidity order of phenol (I), 4-hydroxybenzaldehyde (II), and 3-hydroxybenzaldehyde (III) is:

• A. I<II<III
• B. I<III<II
• C. II<I<III
• D. III<I<II

Answer 1

The Correct Option is B

Phenol is acidic due to the resonance stabilization of the phenoxide ion. The introduction of an electron-withdrawing group such as -CHO increases acidity by stabilizing the negative charge on the oxygen atom.

• 4-Hydroxybenzaldehyde (II): The -CHO group at the para position exerts the maximum electron-withdrawing effect due to resonance and inductive effects, making II the most acidic.
• 3-Hydroxybenzaldehyde (III): The -CHO group at the meta position provides a lesser electron-withdrawing effect compared to the para position, making III less acidic than II but more acidic than I.
• Phenol (I): Without any substituents, phenol has the lowest acidity among the three compounds.

Therefore, the acidity order is I<III<II.