To determine the acidity order, we need to consider the effects of the hydroxyl groups on the acidity of the benzoic acid.
Step 1: Draw the Structures
Let's draw the structures of the three compounds:
Salicylic acid (2-hydroxybenzoic acid): The hydroxyl group is in the ortho position.
4-hydroxybenzoic acid: The hydroxyl group is in the para position.
2,6-dihydroxybenzoic acid: There are two hydroxyl groups in the ortho positions.
Step 2: Factors Affecting Acidity
Several factors influence the acidity of organic acids:
Inductive effect: Electron-withdrawing groups increase acidity, while electron-donating groups decrease acidity.
Resonance effect: Groups that stabilize the conjugate base increase acidity.
Ortho effect: Substituents in the ortho position can have a significant effect on acidity due to steric and electronic interactions. This is complex and not predictable just by looking at inductive and resonance effect.
Intramolecular hydrogen bonding: Can stabilize the conjugate base (increase acidity) or the acid itself (decrease acidity).
Step 3: Acidity Analysis of the Given Compounds
4-hydroxybenzoic acid:
The hydroxyl group in the para position has a weaker effect on acidity because it is farther from the carboxyl group. The -OH group exhibits both +R (resonance) and -I (inductive) effects. Usually, the +R effects counteract each other, and the -I effect slightly decreases the acidity of the compound relative to benzoic acid.
Salicylic acid (2-hydroxybenzoic acid):
The hydroxyl group in the ortho position has a stronger effect than in the para position. The ortho effect and intramolecular hydrogen bonding influence acidity. Salicylic acid benefits from intramolecular hydrogen bonding which stabilises the conjugate base, increasing its acidity.
2,6-dihydroxybenzoic acid:
The presence of two hydroxyl groups in the ortho positions further increases the acidity. The effect of one hydroxyl group in ortho is stabilising the conjugate base through hydrogen bonding. With two OH groups, more hydrogen bonding is possible, stabilising the anion more than in salicylic acid.
Step 4: Putting It All Together
Based on the analysis, we can conclude the following acidity order:
