Acidity of Aromatic Acids - Key Factors Influencing Acidity

The acidity of aromatic acids is determined by the ability of the molecule to donate a proton (H⁺) from the carboxyl group.
After losing a proton, the molecule forms a carboxylate anion (Ar-COO⁻).
The stability of this anion is a key factor in determining the acid strength.

Resonance Stabilization:
- In aromatic acids like benzoic acid, the carboxylate anion (C₆H₅COO⁻) is stabilized by resonance.
- The negative charge is delocalized over the oxygen atoms, making the acid more acidic compared to aliphatic carboxylic acids where resonance is less significant.
Inductive Effect:
- The benzene ring can also exert an inductive effect, which can either increase or decrease acidity depending on the nature of substituents on the ring.

Effect of Substituents on Acidity

Substituents on the aromatic ring significantly affect the acidity of aromatic acids, such as benzoic acid.
The effect depends on whether the substituent is electron-withdrawing or electron-donating.

Electron-Withdrawing Groups (EWGs):
- Examples: -NO₂, -CN, -CHO, -COOH, -SO₃H, -Cl, -Br
- Effect: EWGs increase acidity by stabilizing the carboxylate anion through inductive or resonance effects, making the anion more stable.
- Example: p-Nitrobenzoic acid is more acidic than benzoic acid due to the strong electron-withdrawing nature of the -NO₂ group.
Electron-Donating Groups (EDGs):
- Examples: -CH₃, -OCH₃, -NH₂, -OH
- Effect: EDGs decrease acidity by donating electron density, destabilizing the carboxylate anion.
- Example: p-Methoxybenzoic acid is less acidic than benzoic acid because the -OCH₃ group donates electron density.
Position of Substituents:
- Ortho Effect: Substituents in the ortho position can increase acidity due to steric hindrance, affecting the resonance and stability of the carboxylate anion.