To determine the correct acidity order of phenol (I), 4-hydroxybenzaldehyde (II), and 3-hydroxybenzaldehyde (III), we need to consider the electron-withdrawing or electron-donating effects of the substituents on the phenol ring and how these affect the stability of the phenoxide ion after deprotonation.
Phenol (I) has the structure:
In 4-hydroxybenzaldehyde (II), the aldehyde group (CHO) is at the para position relative to the hydroxyl group:
In 3-hydroxybenzaldehyde (III), the aldehyde group (CHO) is at the meta position relative to the hydroxyl group:
The presence of the electron-withdrawing aldehyde group influences the acidity of the phenol by stabilizing the negative charge on the oxygen atom of the phenoxide ion. This stabilization is more significant when the aldehyde group is closer (para position) or less steric hindered compared to the hydroxyl group.
1. Phenol (I): Has no additional electron-withdrawing groups to stabilize the phenoxide ion, thus has the least acidity.
4-Hydroxybenzaldehyde (II): The CHO group at the para position significantly withdraws electron density through both inductive and resonance effects, stabilizing the phenoxide ion more effectively than at the meta position.
3-Hydroxybenzaldehyde (III): The CHO group at the meta position primarily withdraws electron density through inductive effects, which is less stabilizing than the para position incorporation.
Therefore, the correct acidity order is:
Phenol (I) < 3-Hydroxybenzaldehyde (III) < 4-Hydroxybenzaldehyde (II)
This corresponds to Option B:
I<III<II