Phenol is more acidic than ethyl alcohol primarily because the conjugate base formed after losing a proton, called the phenoxide ion, is resonance stabilized. This means that the negative charge on the oxygen atom in phenoxide ion is delocalized over the aromatic ring, making the ion much more stable. In contrast, the conjugate base of ethyl alcohol, the ethoxide ion, has a localized negative charge on oxygen without resonance stabilization, making it less stable and thus less acidic. Key reasons include:
- In phenol, the −OH group is attached to an sp2 hybridized carbon of the aromatic ring, which is more electronegative, decreasing electron density on oxygen and increasing O−H bond polarity, favoring proton release.
- The negative charge in the phenoxide ion is spread over multiple atoms in the benzene ring by resonance, enhancing stability.
- In ethyl alcohol, the −OH group is attached to an sp3 hybridized carbon, and the ethoxide ion's negative charge remains localized on oxygen, making it less stable.
- Electron-donating alkyl groups in alcohols destabilize their conjugate base by pushing electron density, whereas the aromatic ring in phenol allows delocalization.
- As a result, phenol ionizes more readily than ethyl alcohol, making phenol a stronger acid.
Thus, the resonance stabilization of the phenoxide ion and the higher electronegativity environment around the −OH group in phenol account for its greater acidity compared to ethyl alcohol.
