Reactions of phenols

The –OH group makes the benzene ring of phenol much more reactive than benzene. Its oxygen pushes electrons into the ring by resonance, so the ortho and para positions become rich in electrons. Electrophiles attack there, and they do it easily.

Nitration. With dilute nitric acid at about 298 K, phenol gives a mixture of o-nitrophenol and p-nitrophenol. They are separated by steam distillation: the ortho isomer is steam volatile because its intramolecular hydrogen bond keeps it from bonding to water. With concentrated nitric acid, all three ortho and para positions are nitrated, giving 2,4,6-trinitrophenol, known as picric acid.

Bromination. Phenol reacts with bromine without the iron catalyst that benzene needs.

• With bromine in a non-polar solvent (CS₂ or CHCl₃) at low temperature, only one Br goes on, mostly at the para position.
• With bromine water, the ring is so activated that all three positions react at once, giving a white precipitate of 2,4,6-tribromophenol. This is a test for phenol.

Kolbe's reaction. Sodium phenoxide reacts with carbon dioxide under pressure; acidifying then gives 2-hydroxybenzoic acid, salicylic acid, the starting material for aspirin.

Reimer-Tiemann reaction. Phenol is heated with chloroform and aqueous NaOH; acidifying gives 2-hydroxybenzaldehyde, salicylaldehyde. A –CHO group goes in at the ortho position.

Two more: heating phenol with zinc dust removes the oxygen and gives benzene; oxidising it with chromic acid gives benzoquinone.