Why is phenol acidic? Explain what happens to the acidity of phenols when an electron donating or an electron withdrawing group is attracted to phenol

Answer 1

Phenol is considered acidic primarily due to the presence of a hydroxyl group (-OH) attached to an aromatic benzene ring. The acidity of phenol can be explained by the stability of the phenoxide ion (C₆H₅O⁻) that forms when phenol donates a proton (H⁺). When the -OH group releases a proton, the resulting phenoxide ion is stabilized by resonance. The negative charge on the oxygen can be delocalized across the aromatic ring, leading to several resonance structures:

1. The negative charge can be distributed over the ortho and para positions of the aromatic ring.
2. This resonance delocalization stabilizes the negative charge, making the removal of a proton more favorable and thus increasing the acidity of phenol.

Effect of Substituents on the Acidity of Phenols

1. Electron-Withdrawing Groups (EWGs):

   • Electron-withdrawing groups (such as nitro (-NO₂), cyano (-CN), carbonyl (C=O), or halogens) increase the acidity of phenols.
   • These groups stabilize the phenoxide ion by pulling electron density away from the aromatic system through inductive or resonance effects.
   • For example, a nitro group at the para position can resonate with the negative charge of the phenoxide ion, stabilizing it and making it easier for phenol to lose a proton.
   • As a result, the presence of substituents such as -NO₂ increases the acidity of phenols.

2. Electron-Donating Groups (EDGs):

   • Electron-donating groups (such as alkyl groups, -OCH₃, -NH₂, or -OH) decrease the acidity of phenols.
   • These groups push electron density into the aromatic ring, which increases the electron density on the oxygen in the hydroxyl group, thus destabilizing the phenoxide ion.
   • The resultant phenoxide ion will be less stable because additional electron density is destabilizing in the ion, making it less favorable for phenol to lose a proton.
   • For instance, the presence of a methoxy group (-OCH₃) at the para position makes the phenol less acidic compared to unsubstituted phenol.

In summary, the acidity of phenols is influenced by substituents attached to the aromatic ring: electron-withdrawing groups enhance acidity while electron-donating groups diminish it.