Acidity of Phenols

Phenols are the organic compounds containing benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. Phenols react with active metals like sodium, 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 acidity of phenols is higher in comparison to the alcohols and water molecules.

Explanation

The acidity of phenols is due to its ability to lose hydrogen ion to form phenoxide ions. In a phenol molecule, the sp2hybridised carbon atom of benzene ring attached directly to the hydroxyl group acts as an electron withdrawing group. This sp2 hybridized carbon atom of benzene ring attached directly to the hydroxyl group has higher electronegativity in comparison to hydroxyl group. Due to the higher electronegativity of this carbon atom in comparison to the hydroxyl group attached, electron density decreases on 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 negative charge due to the resonance in benzene ring. Phenoxide ion has greater stability than phenols, as in case of phenol charge separation takes place during resonance.

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The resonance structures of phenoxide ions explain the delocalization of negative charge. In case of substituted phenols, acidity of phenols increases in the presence of 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 ortho and para positions of the attached benzene ring. On the other hand, the acidity of phenols decreases in presence of electron donating groups as they prohibit the formation of phenoxide ion.

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