Reactions of phenols

–OH makes the ring more reactive to electrophiles and sends the new group to the ortho and para positions, the carbons whose electron density resonance raises.

With dilute nitric acid at 298 K phenol gives a mixture of ortho- and para-nitrophenols. Steam distillation separates them: the ortho isomer is volatile because of intramolecular hydrogen bonding, while the para isomer is held back by intermolecular hydrogen bonds.

Concentrated nitric acid converts phenol to 2,4,6-trinitrophenol (picric acid), but the yield is poor. It is now made by sulphonating phenol to phenol-2,4-disulphonic acid first and then treating that with concentrated nitric acid. Picric acid is a strong acid.

Bromination needs no Lewis acid because the –OH makes the ring so reactive. Bromine in a solvent of low polarity (CHCl₃ or CS₂) at low temperature gives monobromophenols.

With bromine water, phenol gives a white precipitate of 2,4,6-tribromophenol.

Kolbe's reaction: phenoxide (from phenol and NaOH) is even more reactive than phenol, so it is substituted even by the weak electrophile CO₂; ortho-hydroxybenzoic acid is the main product.

Reimer–Tiemann reaction: phenol with chloroform and sodium hydroxide gains a –CHO group at the ortho position. The intermediate, a substituted benzal chloride, is hydrolysed by alkali to salicylaldehyde.

Heating phenol with zinc dust converts it to benzene. Chromic acid oxidises phenol to benzoquinone, and phenols left in air slowly darken as quinones form.