To determine which compound among the given options has the lowest \( pK_a \) value, we need to understand the impact of substituents on the acidity of benzoic acid derivatives.
The \( pK_a \) value is inversely related to the acidity of a compound: a lower \( pK_a \) means a stronger acid.
Benzoic acids usually have different substituents attached to the benzene ring, influencing the acidity through electronic effects.
The o-hydroxy benzoic acid (salicylic acid) is expected to have the lowest \( pK_a \) among the given compounds due to intramolecular hydrogen bonding.
Intramolecular hydrogen bonding in o-hydroxy benzoic acid stabilizes the negative charge on the carboxylate group once the proton is removed. This stabilization enhances acidity, lowering the \( pK_a \).
In contrast, m-hydroxy benzoic acid and p-hydroxy benzoic acid do not have this intramolecular hydrogen bonding due to their positions relative to the hydroxyl group, leading to less effective charge stabilization.
Benzoic acid, without any additional groups to stabilize the carboxylate ion, will generally have a higher \( pK_a \) value than the ortho-substituted compound.
Based on these observations, the o-hydroxy benzoic acid is the most acidic due to the increased stability of its conjugate base via intramolecular hydrogen bonding.
