Structure, Acidity & Physical Properties

Structure & Resonance: $$-\text{OH}$$ group attached to $$sp^2$$ hybridized carbon of benzene ring. Resonance increases electron density at ortho and para positions.

Acidity Comparison:

$$\text{Carboxylic Acid} > \text{Phenol} > \text{Water} > \text{Alcohol}$$

• Phenol is acidic because the phenoxide ion ($$\text{C}_6\text{H}_5\text{O}^-$$) is resonance-stabilized.
• Effect of Substituents on Acidity:
  • Electron Withdrawing Groups (EWG: $$-\text{NO}_2, -\text{CN}, -\text{X}$$): Increase acidity (Picric acid, $$2,4,6\text{-trinitrophenol}$$, is highly acidic).
  • $$\text{Acidity Order: } p\text{-nitrophenol} > m\text{-nitrophenol} > \text{phenol}$$
  • Electron Donating Groups (EDG: $$-\text{CH}_3, -\text{OCH}_3$$): Decrease acidity.