Acidity of Phenols

Phenols are organic compounds containing a benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. Phenols react with active metals like sodium, and potassium to form phenoxide. This reaction of phenol with metals indicates its acidic nature.

Phenols react with aqueous sodium hydroxide too to produce phenoxide ions. This indicates that the acidity of phenols is higher in comparison to the alcohols and water molecules.

The Explanation for the Acidity of Phenols

The acidity of phenols is due to their ability to lose hydrogen ions to form phenoxide ions.

• In a phenol molecule, the sp²hybridised carbon atom of the benzene ring attached directly to the hydroxyl group acts as an electron-withdrawing group.
• This sp² hybridized carbon atom of a benzene ring attached directly to the hydroxyl group has higher electronegativity in comparison to the hydroxyl group.
• Due to the higher electronegativity of this carbon atom in comparison to the hydroxyl group attached, electron density decreases on the oxygen atom.
• The decrease in electron density increases the polarity of O-H bond and results in the increase in ionization of phenols.
• Thus, the phenoxide ion is formed. The phenoxide ion formed is stabilized by the delocalization of the negative charge due to the resonance in the benzene ring.
• Phenoxide ion has greater stability than phenols, as in the case of phenol charge separation takes place during resonance.

Acidity of Phenols

The resonance structures of phenoxide ions explain the delocalization of negative charge. In the case of substituted phenols, the acidity of phenols increases in the presence of the electron-withdrawing group. This is due to the stability of the phenoxide ion generated. The acidity of phenols further increases if these groups are attached at ortho and para positions.

This is due to the fact that the negative charge in phenoxide ion is mainly delocalized at the ortho and para positions of the attached benzene ring. On the other hand, the acidity of phenols decreases in the presence of electron-donating groups as they prohibit the formation of phenoxide ions.

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