Properties of phenols

The essentials

Phenols are more acidic than alcohols because the phenoxide ion is stabilised by resonance: its negative charge is delocalised over the benzene ring. Phenol reacts with NaOH to form sodium phenoxide, but unlike carboxylic acids it does not liberate CO₂ from sodium hydrogencarbonate.

Key points

• Effect of substituents: electron-withdrawing groups (e.g. –NO₂) at ortho and para positions increase acidity; electron-releasing groups (e.g. –CH₃ in cresols) decrease it. p-Nitrophenol (pKa about 7.1) is more acidic than phenol (about 10.0), which is far more acidic than ethanol (about 15.9).
• The –OH group activates the ring and directs electrophiles to ortho and para positions.
• Nitration: dilute HNO₃ at low temperature gives o- and p-nitrophenol; they are separated by steam distillation since o-nitrophenol is steam volatile due to intramolecular hydrogen bonding. Concentrated HNO₃ gives 2,4,6-trinitrophenol (picric acid).
• Bromination: bromine water gives a white precipitate of 2,4,6-tribromophenol; Br₂ in CS₂ at low temperature gives mainly monobromophenols.
• Kolbe's reaction: sodium phenoxide + CO₂, then acid, gives salicylic acid (2-hydroxybenzoic acid).
• Reimer–Tiemann reaction: phenol + CHCl₃ + NaOH, then acid, gives salicylaldehyde (2-hydroxybenzaldehyde).
• Reduction: heating phenol with zinc dust gives benzene.
• Oxidation: with chromic acid, phenol gives benzoquinone; phenols also darken slowly in air.

Common mistakes

• Saying phenol dissolves in NaHCO₃ with effervescence; that test is for carboxylic acids.