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Acidity of Phenols

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  • Phenols exhibit acidic properties due to the hydroxyl group (-OH) attached to the aromatic ring.

  • The acidity arises from the ability of the hydroxyl hydrogen to dissociate as a proton (H⁺), forming a phenoxide ion (C₆H₅O⁻).

  • This acidity is primarily due to the resonance stabilization of the phenoxide ion, where the negative charge on the oxygen is delocalized over the benzene ring.

Factors Affecting the Acidity of Phenols

Resonance Stabilization

  • The phenoxide ion is stabilized by resonance, making phenols more acidic than alcohols, which lack such stabilization.

Electron-Withdrawing Substituents (EWGs)

  • EWGs (e.g., -NO₂, -X, -C=O) increase phenol acidity by stabilizing the phenoxide ion through electron withdrawal, especially when positioned close to the hydroxyl group.

Electron-Donating Substituents (EDGs)

  • EDGs (e.g., alkyl groups, -NH₂) decrease phenol acidity by destabilizing the phenoxide ion through electron donation, making it harder for phenol to lose a proton.

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