9 min readPathway sheet2-mark equations for each method; the cumene process as a 3-mark favourite; conversions in 5-mark answers

Preparation of phenols

Phenol is made from chlorobenzene, benzenesulphonic acid, diazonium salts or cumene. The cumene route also gives acetone, a valuable by-product.

From chlorobenzeneC₆H₅Cl + 2NaOH → C₆H₅ONa + NaCl + H₂O (623 K, 300 atm); acid then gives phenol

From a diazonium saltC₆H₅N₂⁺Cl⁻ + H₂O (warm) → C₆H₅OH + N₂ + HCl

CumeneIsopropylbenzene, C₆H₅CH(CH₃)₂; air oxidation gives cumene hydroperoxide

Cumene hydroperoxide + dilute acid→ phenol + propanone (acetone)

From haloarenes

1★
C₆H₅Cl + 2NaOH → C₆H₅ONa + NaCl + H₂O
NaOH at 623 K and 300 atm (the text of this chapter says 320 atm; its figure and the haloalkanes chapter say 300 atm). The C-Cl bond is strong, so the conditions are drastic
2
C₆H₅ONa + HCl → C₆H₅OH + NaCl
Acidifying sodium phenoxide gives phenol

★ key step

From benzenesulphonic acid

1
C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O
Sulphonation with oleum (fuming sulphuric acid)
2★
C₆H₅SO₃H + 3NaOH → C₆H₅ONa + Na₂SO₃ + 2H₂O
Heat with molten NaOH
3
C₆H₅ONa + HCl → C₆H₅OH + NaCl
Acidify

★ key step

From diazonium salts

1
C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O
Diazotisation at 273 to 278 K: an aromatic primary amine reacts with nitrous acid (made from NaNO₂ and HCl)
2★
C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl
Warm with water, or treat with dilute acid; nitrogen gas is given off

★ key step

From cumene

1
C₆H₅CH(CH₃)₂ + O₂ → C₆H₅C(CH₃)₂OOH
Cumene is oxidised by air to cumene hydroperoxide. (Cumene itself is made from benzene and propene.)
2★
C₆H₅C(CH₃)₂OOH → C₆H₅OH + CH₃COCH₃
Treat with dilute acid: it splits into phenol and propanone (acetone)

★ key step

Why the cumene process matters

Phenol was first isolated from coal tar in the early nineteenth century; today it is made synthetically. Most of the world's phenol comes from cumene, because one cheap process gives two useful products: phenol and acetone, a widely used solvent.

In real life

Phenol is the raw material for Bakelite, the hard plastic in electrical switches and pressure-cooker handles. The acetone made alongside it in the cumene process is the solvent in many nail-polish removers.

Exam angle

3 marks: “A plant makes phenol from cumene and also sells a second product. Write the two steps with conditions, and name the second product.” C₆H₅CH(CH₃)₂ + O₂ (air) → C₆H₅C(CH₃)₂OOH (cumene hydroperoxide). With dilute acid → C₆H₅OH + CH₃COCH₃. By-product: acetone (propanone).

2 marks: “Convert (i) chlorobenzene into phenol and (ii) aniline into phenol.” (i) C₆H₅Cl + 2NaOH → C₆H₅ONa + NaCl + H₂O (623 K, 300 atm); C₆H₅ONa + HCl → C₆H₅OH + NaCl. (ii) C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O (273 to 278 K); C₆H₅N₂⁺Cl⁻ + H₂O (warm) → C₆H₅OH + N₂ + HCl.

2 marks: “Write the preparation of phenol from benzenesulphonic acid.” C₆H₅SO₃H + 3NaOH (molten) → C₆H₅ONa + Na₂SO₃ + 2H₂O; then C₆H₅ONa + HCl → C₆H₅OH + NaCl.

Remember it

Four roads to phenol: Chloro, Sulpho, Diazo, Cumene. Cumene gives a bonus: acetone.