The highest acidic compound among the following is _______.
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When comparing substituted benzoic acids, look for the "ortho effect." Ortho substituents, especially those capable of hydrogen bonding (like -OH, -NH\(_2\)), can have a surprisingly large impact on acidity due to steric hindrance and intramolecular H-bonding that stabilizes the conjugate base.
Step 1: Analyze the effect of substituents on the acidity of benzoic acid.
Acidity is determined by the stability of the conjugate base (carboxylate anion, -COO\(^-\)). Electron-withdrawing groups (EWGs) stabilize the anion and increase acidity. Electron-donating groups (EDGs) destabilize the anion and decrease acidity.
Step 2: Evaluate each compound.
- Benzoic acid: This is our reference compound.
- o-Hydroxybenzoic acid (Salicylic acid): The -OH group is an EWG via its inductive effect (-I) but an EDG via resonance (+R). However, at the ortho position, the conjugate base is significantly stabilized by intramolecular hydrogen bonding between the -OH proton and the -COO\(^-\) group. This is a powerful stabilizing effect known as the "ortho effect," making it much more acidic than benzoic acid.
- o-Aminobenzoic acid: The -NH\(_2\) group is a strong EDG via resonance (+R), which destabilizes the conjugate base and decreases acidity.
- p-Methoxybenzoic acid: The -OCH\(_3\) group at the para position is a strong EDG via resonance (+R), destabilizing the conjugate base and making it less acidic than benzoic acid.
Step 3: Compare the acidities.
Due to the strong stabilization from intramolecular hydrogen bonding, o-hydroxybenzoic acid is the most acidic compound among the choices.
