The acidity of phenol is influenced by substituents on the aromatic ring, which either increase or decrease acidity based on their electronic effects.
Electron-Withdrawing Groups (EWGs):
Examples: Nitro (-NO₂), cyano (-CN), carboxyl (-COOH), halogens (-Cl, -Br).
Effect: Increase acidity.
Mechanism: EWGs stabilize the phenoxide ion by withdrawing electron density through inductive or resonance effects, making it easier for phenol to lose a proton.
Position Dependence:
Ortho- and Para- Positions: Strongly increase acidity via resonance.
Meta- Position: Less pronounced effect, primarily inductive.
Electron-Donating Groups (EDGs) (Acidity of Phenol):
Examples: Alkyl (-CH₃), amino (-NH₂), hydroxyl (-OH), alkoxy (-OCH₃).
Effect: Decrease acidity.
Mechanism: EDGs donate electron density, destabilizing the phenoxide ion and making it harder for phenol to lose a proton.
Position Dependence:
Ortho- and Para- Positions: Significantly reduce acidity via resonance.
Meta- Position: Less effect, primarily inductive.
Example: p-Cresol (4-methylphenol) is less acidic than phenol due to the methyl groups.