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's acidity stems from the resonance stabilization of its phenoxide ion. Adding an electron-withdrawing group like -CHO enhances acidity by stabilizing the oxygen's negative charge.

• 4-Hydroxybenzaldehyde (II): The para-positioned -CHO group maximizes electron withdrawal due to resonance and induction, making II the most acidic.
• 3-Hydroxybenzaldehyde (III): The meta-positioned -CHO group's electron withdrawal is weaker than at the para position, placing III between II and I in acidity.
• Phenol (I): Lacking substituents, phenol exhibits the lowest acidity.

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