Chemical properties

1. Acidity — phenol compared with alcohols. Both react with sodium metal releasing hydrogen: 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂↑ ; 2ROH + 2Na → 2RONa + H₂↑. But only phenol reacts with sodium hydroxide solution (alcohols do not): C₆H₅OH + NaOH → C₆H₅ONa + H₂O (sodium phenoxide). Hence phenol is called carbolic acid. The benzene ring withdraws electrons from the oxygen of the O–H group, which makes the O–H bond more polar, longer and weaker, and so the proton leaves more easily. (Phenol is still a weak acid – weaker than carboxylic acids and even carbonic acid.)

• 2.
• Reaction with halogen acids. In alcohols the –OH can be replaced by X with HX.
• In phenol the C–O bond is shorter and stronger (the oxygen lone pair interacts with the ring), so the –OH cannot be replaced by halogen acids; phenol does not react with HCl.

3. Nitration. Phenol with concentrated nitric acid in the presence of concentrated sulphuric acid gives 2,4,6-trinitrophenol (picric acid): C₆H₅OH + 3HNO₃ —(conc. H₂SO₄)→ C₆H₂(NO₂)₃OH + 3H₂O Picric acid is an explosive and is used in treating burns; it stains the skin yellow for several days (until the outer skin layer is renewed).

4. Condensation with formaldehyde → Bakelite. In acidic or basic medium phenol reacts with methanal (formaldehyde, HCHO): one molecule of methanal joins two molecules of phenol with loss of water; the units keep joining by condensation polymerisation to form a three-dimensional network polymer, Bakelite. Condensation polymers are formed from two kinds of monomers with elimination of a small molecule such as H₂O. Bakelite is a dark-brown thermosetting plastic (it does not soften again on heating), heat-resistant and a good electrical insulator — used for electrical fittings, switches, handles and ash trays.