A phenoxide ion is the conjugate base of phenol.
- Phenoxide is a conjugate base, this implies that it is formed from an acid that has given up its hydrogen.
- This acid is a phenol molecule.
- The hydrogen of the hydroxyl leaves, and an O− remains, forming the ‘oxide ion’ part of the phenoxide ion.
Acidity of Phenols due to Phenoxide ions
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 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 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.
Resonance of Phenoxide ions
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.
- The reason behind the fact is 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 the presence of electron-donating groups as they prohibit the formation of phenoxide ion.